docs(bidders): remove obsolete pgam + pgammedia pages

Both entries are stale:
- pgam: biddercode 'pgam' has no active adapter in prebid-js master
- pgammedia: was an aniview alias page; the live PGAM SSP is
  documented at pgamssp.md and pgamdirect.md

Round-3 pre-launch QA (FND-008) flagged these as dead listings that
misdirect publisher integrators. Removing to leave only the two
active adapter docs.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: mastap150