tarfs: remove `ReadFile`

[BradLugo]

Description

Complete removes ReadFile from tarfs.

Follow up to https://github.com/quay/claircore/pull/1305.

[BradLugo]

I wanted to use this PR to discuss the merits of removing ReadFile entirely from tarfs (see my comment here for more context: https://github.com/quay/claircore/pull/1305#issuecomment-2046286920).

If there is a significant reason to keep ReadFile around, I'll change this PR to expand on testing for it since there are only 1-2 tests currently.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 56.25%. Comparing base (e31accc) to head (94e3ee6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1325      +/-   ##
==========================================
- Coverage   56.28%   56.25%   -0.04%     
==========================================
  Files         266      266              
  Lines       16842    16812      -30     
==========================================
- Hits         9480     9458      -22     
+ Misses       6398     6391       -7     
+ Partials      964      963       -1     

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[hdonnay]

/fast-forward