Revert "Readd Git backend, again"

[beckermr]

Reverts #2795

I am reverting this PR again since the bot is not making PRs against repos it had not forked previously.

[codecov]

Codecov Report

Attention: Patch coverage is 22.48062% with 100 lines in your changes missing coverage. Please review.

Project coverage is 75.23%. Comparing base (2034547) to head (e9ca405).

Files	Patch %	Lines
conda_forge_tick/git_utils.py	9.52%	95 Missing ⚠️
conda_forge_tick/auto_tick.py	0.00%	3 Missing ⚠️
conda_forge_tick/executors.py	91.66%	1 Missing ⚠️
conda_forge_tick/update_prs.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2899      +/-   ##
==========================================
- Coverage   76.95%   75.23%   -1.72%     
==========================================
  Files         112      112              
  Lines       11950    11367     -583     
==========================================
- Hits         9196     8552     -644     
- Misses       2754     2815      +61     

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[beckermr]

I'm going to spend a few minutes trying to debug before merging this.