[X86][NFC] Remove unused encoding string: VK[1/2/4/8/16]PAIR

This is also a bug. The VK[1/2/4/8/16]PAIR here should be VK[1/2/4/8/16]Pair which has its
custom PrintMethod and ParserMatchClass. However we don't have any instructions using vvvv
and ModR/M.REG field so this issue is not exposed.

Differential Revision: https://reviews.llvm.org/D109564