[AMDGPU] Removed unused method

Summary:
I accidentally left this behind in D50306, and it causes a build warning
when I build with gcc7.

Subscribers: arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D52022

Change-Id: I30f7a47047e9d9d841f652da66d2fea19e74842c
llvm-svn: 342189