Fix memory issue in AttrBuilder::removeAttribute uses.

Summary:
Found when running Valgrind.

This removes two unnecessary assignments when using
AttrBuilder::removeAttribute.

AttrBuilder::removeAttribute returns a reference to the object.
As the LHSes were the same as the callees, the assignments
resulted in memcpy calls where dst = src.

Commited on behalf-of: dstenb (David Stenberg)

Reviewers: mkuper, rnk

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D25460

llvm-svn: 285298