- Only two audits were using this: `audit_keg_only_reason` and `audit_text`,
and they weren't using any of its text processing methods, so there's little
reason to keep it around.
- The "`keg_only_reason` shouldn't contain 'HOMEBREW_PREFIX'" audit can easily
be replaced with a RuboCop since that's "just" text parsing.
- The "tests should invoke binaries with `bin/<command>`" audit had to stay as
a FormulaAudit because it requires accessing attributes about the Formula
like its name, aliases, which RuboCop can't get to, but it was easy to move the
singular "read the text in the file" line from `FormulaTextAuditor`.
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