[X86] remove derived method w/ same impl as base

Summary:
While looking into issues with IfConverter, I noticed that
X86InstrInfo::isUnpredicatedTerminator matched its overriden
implementation in TargetInstrInfo::isUnpredicatedTerminator.

Reviewers: craig.topper, hfinkel, MaskRay, echristo

Reviewed By: MaskRay, echristo

Subscribers: hiraditya, llvm-commits, srhines

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62749