Remove deprecated code

[IKarbowiak]

Drop deprecated code that was marked for being removed in 2.10.

Pull Request Checklist

1. Privileged views and APIs are guarded by proper permission checks.
2. All visible strings are translated with proper context.
3. All data-formatting is locale-aware (dates, numbers, and so on).
4. Database queries are optimized and the number of queries is constant.
5. Database migration files are up to date.
6. The changes are tested.
7. GraphQL schema and type definitions are up to date.
8. Changes are mentioned in the changelog.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@c47336f). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff            @@
##             master   #5107   +/-   ##
========================================
  Coverage          ?     91%           
========================================
  Files             ?     259           
  Lines             ?   16751           
  Branches          ?    1489           
========================================
  Hits              ?   15245           
  Misses            ?    1103           
  Partials          ?     403

Impacted Files	Coverage Δ
saleor/product/utils/availability.py	95.34% <ø> (ø)
saleor/product/utils/costs.py	81.81% <ø> (ø)
saleor/graphql/product/resolvers.py	85.32% <100%> (ø)
saleor/search/backends/postgresql_storefront.py	100% <100%> (ø)
saleor/graphql/checkout/mutations.py	94.15% <100%> (ø)
saleor/graphql/discount/filters.py	90% <100%> (ø)

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[django-queries]

Here is the report for ce15c4c (mirumee/saleor @ remove-deprecated-code)
Base comparison is e8416de.

**Found 2 differences!** (click me)

# api.benchmark checkout
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  add billing address to checkout            	         34	         34	             20
  add shipping to checkout                   	          7	          7	              0
  checkout payment charge                    	         10	         10	              0
  complete checkout                          	          8	          8	              0
  create checkout                            	         50	         50	             24

# api.benchmark homepage
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  retrieve main menu                         	          5	          5	              0
  retrieve product list                      	          4	          4	              0
  retrieve secondary menu                    	          5	          5	              0
  retrieve shop                              	          2	          2	              0

# api.benchmark product
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
+ product details                            	         15	         14	              2
  retrieve product attributes                	          9	          9	              0

# api.benchmark variant
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  product variant bulk create                	         51	         51	              3
- retrieve variant list                      	         15	         16	              6

# api product sorting attributes
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  sort product not having attribute data     	         21	         21	              0