MNT: Don't call get_interface_groups with include_plugins=True

[kyleam]

Calling get_interface_groups() with include_plugins=True was marked as
deprecated in gh-5554.

Update: And also remove datalad.locations.{system,user}-plugins options.

[codecov]

Codecov Report

Merging #5564 (a98e1b5) into master (f937558) will decrease coverage by 1.82%.
The diff coverage is 100.00%.

❗ Current head a98e1b5 differs from pull request most recent head b94d0f4. Consider uploading reports for the commit b94d0f4 to get more accurate results

@@            Coverage Diff             @@
##           master    #5564      +/-   ##
==========================================
- Coverage   90.22%   88.39%   -1.83%     
==========================================
  Files         308      305       -3     
  Lines       42516    42480      -36     
==========================================
- Hits        38361    37552     -809     
- Misses       4155     4928     +773     

Impacted Files	Coverage Δ
datalad/interface/common_cfg.py	100.00% <ø> (ø)
datalad/api.py	67.39% <100.00%> (ø)
datalad/cmdline/main.py	74.89% <100.00%> (ø)
datalad/distribution/dataset.py	94.40% <100.00%> (ø)
datalad/plugin/wtf.py	0.00% <0.00%> (-100.00%)	⬇️
datalad/plugin/addurls.py	0.00% <0.00%> (-100.00%)	⬇️
datalad/plugin/no_annex.py	0.00% <0.00%> (-100.00%)	⬇️
datalad/plugin/add_readme.py	0.00% <0.00%> (-100.00%)	⬇️
datalad/plugin/check_dates.py	0.00% <0.00%> (-100.00%)	⬇️
datalad/support/tests/utils.py	0.00% <0.00%> (-100.00%)	⬇️
... and 68 more

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[mih]

Crippled tests show #5300 and brew is still having issues installing git-annex.

[mih]

Thx for the clean-up!