[analyzer] Remove the unused method declaration in `ValistChecker.cpp`.

Summary: `getVariableNameFromRegion()` seems useless.

Reviewers: xazax.hun, george.karpenkov

Reviewed By: xazax.hun

Subscribers: szepet, rnkovacs, a.sidorin, cfe-commits, MTC

Differential Revision: https://reviews.llvm.org/D45081

llvm-svn: 328860