[NVPTXAsmPrinter] clean up dead code. NFC

Summary:
The printOperand function takes a default parameter, for which there are
zero call sites that explicitly pass such a parameter.  As such, there
is no case to support. This means that the method
printVecModifiedImmediate is purly dead code, and can be removed.

The eventual goal for some of these AsmPrinter refactoring is to have
printOperand be a virtual method; making it easier to print operands
from the base class for more generic Asm printing. It will help if all
printOperand methods have the same function signature (ie. no Modifier
argument when not needed).

Reviewers: echristo, tra

Reviewed By: echristo

Subscribers: jholewinski, hiraditya, llvm-commits, craig.topper, srhines

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60727

llvm-svn: 358527