InstrProf: Remove an unnecessary helper function (NFC)

VisitSubStmtRBraceState is really just Visit, as long as
VisitCompoundStatement handles braces correctly.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@221659 91177308-0d34-0410-b5e6-96231b3b80d8