simplify-libcalls: Removed unused variable

The 'Modified' variable should have been removed from SimplifyLibCalls
in r177619, but was missed.  This commit removes it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177622 91177308-0d34-0410-b5e6-96231b3b80d8