[NVPTX] Remove support for `sub` in global initializers. (#201473)
I added this in https://github.com/llvm/llvm-project/pull/201220 and
assumed it worked because the pre-commit builders passed. But (a) this
is not attested in the PTX ISA, and (b) it seems that the builders don't
actually run ptxas.
Oops. Removed support for `sub`.