[MCA] [NFC] Remove unused Index formal from ResourceManager::issueInstruction

Summary: The instruction index was never referenced in the body.  Just a minor cleanup.

Reviewers: andreadb

Reviewed By: andreadb

Subscribers: javed.absar, gbedwell, llvm-commits

Differential Revision: https://reviews.llvm.org/D46142

llvm-svn: 331001