[clang] [RISCV][clang] Remove unused base type in `vfncvt` (PR #101646)

[Pengcheng Wang via cfe-commits]

https://github.com/wangpc-pp approved this pull request.

Make sense to me.
(Do we have a way to test these generated builtins?)

https://github.com/llvm/llvm-project/pull/101646