[flang][NFC] Clean up ConvertType

This patch removes unused or obsolete code in
the ConvertType.h and ConvertType.cpp files. These
files were landed together with the initial flang
upstreaming. This cleanup will help future upstreaming
effort from fir-dev and keep only used code.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D120405