Use run_id_output sorting final_result in multi_run

[dachengx]

What is the problem / what does the code in this PR do

The deleted if statement is not really needed because run_id_output is always generated no matter add_run_id_field is True or False.

Close: #783

Can you briefly describe how it works?

Can you give a minimal working example (or illustrate with a figure)?

Please include the following if applicable:

• Update the docstring(s)
• Update the documentation
• Tests to check the (new) code is working as desired.
• Does it solve one of the open issues on github?

Please make sure that all automated tests have passed before asking for a review (you can save the PR as a draft otherwise).

[coveralls]

coverage: 90.81% (-0.005%) from 90.815%
when pulling ba1d747 on simplify_add_run_id_field
into 04aa884 on master.