Remove legacy oasdiff.yml workflow

The pr-comment.yml (v0.0.34) supersedes oasdiff.yml (v0.0.31). Both
write to the same <!-- oasdiff-pr-report --> comment anchor, so the
old workflow was overwriting the review links added by pr-comment.yml.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: reuvenharrison