Remove unused code related to CPlusPlusLanguage::FindEquivalentNames

Summary: It is simply unused, and the header for it is private, so there should be no external dependencies.

Reviewers: #lldb, zturner

Reviewed By: zturner

Subscribers: zturner, tberghammer, jingham, lldb-commits

Differential Revision: https://reviews.llvm.org/D32503
Patch by Scott Smith <scott.smith@purestorage.com>.

llvm-svn: 301903