Remove unused macro-asm instruction and associated address patching.

On mips64, remove unused JumpLabelToJumpRegister(). On mips, rename
it appropriately (it's still used there for JR->J optimizations).

BUG=

Review URL: https://codereview.chromium.org/1059433003

Cr-Commit-Position: refs/heads/master@{#27593}