[NFC][NVPTX] Remove dead comment and commented code

A confusing comment after the last return statement in
`NVPTXAsmPrinter::doFinalization` referred to a preprocessor macro
(NVISA) that has never existed since the NVPTX backend has been a part
of upstream llvm - as far as the pickaxe will tell me anyway. Thus I've
removed it.

Differential Revision: https://reviews.llvm.org/D140399