Revert "Plugins for flyteadmin server middleware"

[katrogan]

Reverts #420

[codecov]

Codecov Report

Merging #427 (e89bd16) into master (c0e62f4) will decrease coverage by 0.11%.
The diff coverage is n/a.

❗ Current head e89bd16 differs from pull request most recent head 0b8603d. Consider uploading reports for the commit 0b8603d to get more accurate results

@@            Coverage Diff             @@
##           master     #427      +/-   ##
==========================================
- Coverage   61.32%   61.21%   -0.12%     
==========================================
  Files         156      155       -1     
  Lines       11097    11088       -9     
==========================================
- Hits         6805     6787      -18     
- Misses       3588     3597       +9     
  Partials      704      704              

Flag	Coverage Δ
unittests	?

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Impacted Files	Coverage Δ
pkg/rpc/adminservice/base.go	4.12% <ø> (+0.08%)	⬆️
plugins/registry.go	100.00% <ø> (ø)
auth/identity_context.go	0.00% <0.00%> (-29.73%)	⬇️

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