Remove 2 code owner feature flags

Shipping a fix for the web request code owner bugs (see !31283 (merged)) involved adding 2 feature flags, skip_web_ui_code_owner_validations and use_legacy_codeowner_validations. With &3320 (closed) being finished and deployed to www for several weeks, and scheduled for release in %13.2, these 2 flags can also be removed from the code base.