Remove the sample pgo annotation heuristic that uses call count to annotate basic block count.

Summary: We do not need that special handling because the debug info is more accurate now. Performance testing shows no regression on google internal benchmarks.

Reviewers: davidxl, aprantl

Reviewed By: aprantl

Subscribers: llvm-commits, aprantl

Differential Revision: https://reviews.llvm.org/D30658

llvm-svn: 297038