remove support for reading and generating Dvcfile

[skshetry]

Part of #7093.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.30 ⚠️

Comparison is base (3e0f2ba) 91.03% compared to head (51bfc19) 90.73%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9546      +/-   ##
==========================================
- Coverage   91.03%   90.73%   -0.30%     
==========================================
  Files         472      472              
  Lines       36447    36447              
  Branches     5225     4265     -960     
==========================================
- Hits        33179    33072     -107     
- Misses       2701     2785      +84     
- Partials      567      590      +23     

Impacted Files	Coverage Δ
dvc/dvcfile.py	96.04% <100.00%> (-0.02%)	⬇️
dvc/stage/utils.py	96.66% <100.00%> (+0.02%)	⬆️
tests/func/params/test_diff.py	100.00% <100.00%> (ø)
tests/func/params/test_show.py	100.00% <100.00%> (ø)
tests/func/test_repro.py	100.00% <100.00%> (ø)
tests/func/test_run_single_stage.py	100.00% <100.00%> (ø)
tests/unit/stage/test_stage.py	100.00% <100.00%> (ø)

... and 23 files with indirect coverage changes

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