[llvm] [AMDGPU] Get rid of the +wavefrontsizeXX,-wavefrontsizeXX in MC tests (PR #99001)

[Jay Foad via llvm-commits]

jayfoad wrote:

> Now turning off/turning on feature is not needed in most cases. This is NFC, tests only.

What is the behavior now? Specifying either of +wavefrontsize32 or +wavefrontsize64 automatically turns off the other one? But there is still a default if you specify neither, right?

https://github.com/llvm/llvm-project/pull/99001