[NVPTX][AsmPrinter] Avoid removing globals before calling AsmPrinter::doFinalization()

Instead of removing globals from a module, we, it seems, can just override
AsmPrinter::emitGlobalVariable() to do nothing as NVPTXAsmPrinter already
emitted globals by this time and we don't want to do it twice.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D113653