[clang] [RISCV][NFC] Remove unneeded defining name of `vundefined` (PR #101643)

[Brandon Wu via cfe-commits]

4vtomat wrote:

> I don't think this will be better. We may keep these names so that these code can be self-explanatory.

Yeah, it's reasonable to me.

https://github.com/llvm/llvm-project/pull/101643