[CUDA] Use the multi-element remove function in EraseUnwantedCUDAMatches.

Summary:
Bug pointed out by Benjamin Kramer in r264008.  I think the bug is
benign because by the time this is called, we should only have at most
two overloads to consider (either a host and a device overload, or a
host+device overload, but not all three).

Reviewers: tra

Subscribers: cfe-commits, bkramer

Differential Revision: http://reviews.llvm.org/D21914

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@275233 91177308-0d34-0410-b5e6-96231b3b80d8