addSamplingProfileGatherFlags is no longer needed

We already add the "-fdebug-info-for-profiling" flag globally by default.
Besides, Sampling PGO does not need additional target-specific build
flags, unlike instrumentation PGO.

This is preparation to separate out sampling pgo into a separate module.
NFCI.

Test: presubmit
Change-Id: Icb28a98c021a78db25d8cb5ca9694ed6b7367a8d