Remove unused function.

Removing an unused function which is causing one of the build bots to fail.
This was introduced in the commit r223113. A proper cleanup of the so_imm
tblgen defintion (made redundant by the mod_imm definition) needs to happen
soon.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223115 91177308-0d34-0410-b5e6-96231b3b80d8