TableGen: Remove OpInit::Fold

Summary:
Virtual dispatch is not actually used anywhere.

Change-Id: I9829c5c59920ea27fb9bc17f1442156a3bb09a65

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D44197

llvm-svn: 327846