Remove ABIMacOSX_i386::PrepareNormalCall().

Summary:
This was removed from the other ABI plugins long ago. This removes
a warning that was happening in this unused code as a result of
adding 2 new types to Scalar.h (e_uint128 and e_sint128).

Reviewers: clayborg

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12514

llvm-svn: 246608