test(sanitize): drop the optimization scaffolding

The benchmarks and the reference-implementation equivalence harness
existed to justify the sanitizer rewrite. They have served that purpose,
so remove them along with the verbatim copy of the old pipeline they
carried.

Five checks move into sanitize_test.go rather than going away, because
none of them reference the old implementation and all of them guard
behaviour the rewrite introduced:

- isHTMLInert must be a fixed point of the live bluemonday policy,
  checked byte by byte and as whole strings, with the accepted byte set
  pinned explicitly. Nothing else fails if that set is widened, and
  widening it changes sanitizer output.
- Both filters are fixed points on their own output, which is what
  licenses Sanitize to skip its second pass.
- Clean ASCII sanitizes with zero allocations.
- Invalid UTF-8 is re-encoded to U+FFFD.
- Known payloads still lose content.

Net -560 lines.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: SamMorrowDrums