[TII] remove overrides of isUnpredicatedTerminator

Summary:
They all match the base implementation in
TargetInstrInfo::isUnpredicatedTerminator.

Follow up to D62749.

Reviewers: echristo, MaskRay, hfinkel

Reviewed By: echristo

Subscribers: wuzish, nemanjai, hiraditya, kbarton, llvm-commits, srhines

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78976