Remove obsolete code, trim deps on GitPy

[mih]

• remove Repo.is_submodule_modified()
• remove GitRepo._get_added_files_commit_msg()
• strip GitPy from
  • GitRepo.get_indexed_files()
  • GitRepo.get_files()

[codecov]

Codecov Report

Merging #3702 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3702      +/-   ##
==========================================
- Coverage   82.87%   82.84%   -0.03%     
==========================================
  Files         273      273              
  Lines       35809    35794      -15     
==========================================
- Hits        29677    29655      -22     
- Misses       6132     6139       +7

Impacted Files	Coverage Δ
datalad/support/tests/test_gitrepo.py	99.88% <ø> (-0.01%)	⬇️
datalad/support/gitrepo.py	84.52% <100%> (+0.03%)	⬆️
datalad/downloaders/tests/test_http.py	57.77% <0%> (-2.23%)	⬇️

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[kyleam]

Thanks. LGTM.

[mih]

Thx