chore(docs): drop duplicate blog posts left over from mdx migration (#2191)

Two orphans from the gitbook→starlight migration (cc2050c, #2189):
- docs/blog/2026-04-09-why-we-rebuilt-our-ci-pipeline.md — pre-migration
  authoring draft, no longer referenced by mkdocs (mkdocs.yml itself was
  removed in #2189).
- web/content/blog/posts/why-we-rebuilt-our-ci-pipeline-from-40-minutes-to-82-seconds.md
  — duplicate of the shorter-slug Apr 9 post. Keeps the tagged, later-
  edited copy.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: bpamiri