Remove unnecessary cast that was also unnecessarily casting away constness.

Remove unnecessary cast that was also unnecessarily casting away constness.

Even looking at the revision history I couldn't quite piece together why this
cast was ever written in the first place, but I assume it was because of some
change in the inheritance, perhaps this function was reimplemented in a
derived type & this caller was meant to get the base version (& it wasn't
virtual)?

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@162301 91177308-0d34-0410-b5e6-96231b3b80d8