TableGen: Remove VarInit::getFieldType

It is redundant with the implementation in TypedInit.

Change-Id: I8ab1fb5c77e4923f7eb3ffae5889f0f8af6093b4

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D43678

llvm-svn: 326061