Revert "Add StringMap::insert(pair) consistent with the standard associative container concept."

This reverts commit r211309.

It looks like it broke some bots:

http://lab.llvm.org:8011/builders/clang-x86_64-ubuntu-gdb-75/builds/15563/steps/compile/logs/stdio

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211328 91177308-0d34-0410-b5e6-96231b3b80d8