[C++11] Remove a now unnecessary use of std::function for a remove_if
predicate. The wrapper used by SetVector was erroneously requiring an
adaptable predicate. It has been fixed and we really don't want to
require an indirect call for every predicate evaluation.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202744 91177308-0d34-0410-b5e6-96231b3b80d8