[Sanitizer API] Remove SanitizerAPINamespacesForTesting flag.

Remove SanitizerAPINamespacesForTesting flag and support code, to
prepare for a new and different namespace-d content in Sanitizer
proposal.

Ref: WICG/sanitizer-api#181
Bug: 1225606
Change-Id: I13ad92898b28d56224c7a227b21390e927e86a17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4051041
Reviewed-by: Yifan Luo <[email protected]>
Reviewed-by: Koji Ishii <[email protected]>
Reviewed-by: Anders Hartvoll Ruud <[email protected]>
Commit-Queue: Daniel Vogelheim <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1078534}