Rename RISCVMCExpr::VK_RISCV_ to VK_. NFC

They implement relocation operators and are named VK_RISCV_ probably to
avoid confusion with `MCSymbolRefExpr::VariantKind`.
`MCSymbolRefExpr::VariantKind` is discouraged
(https://discourse.llvm.org/t/error-expected-relocatable-expression-with-mctargetexpr/84926/2)
and targets are migrating away from `MCSymbolRefExpr::VariantKind`.

Therefore, there is no need to make the name long in the presence of the
clear `RISCVMCExpr::` prefix.

Pull Request: #131489

Author: MaskRay