[RISCV] Xqcilb: remove RISCVMCExpr::VK_QC_E_JUMP_PLT and drop `@plt` parsing

Follow-up to the just landed #135044 . Remove `@plt` parsing (only
needed by legacy `call foo@plt`). MCParser's `@` parsing is problematic.
Supporting target variations like (`foo+2@plt foo@plt+2 (foo+2)@plt`)
involves messy hacks. We should refrain from adding new `@` uses.

Remove unneeded `RISCVMCExpr::VK_QC_E_JUMP_PLT` (should only be used
when an instruction might have multiple reasonable relocations
https://maskray.me/blog/2025-03-16-relocation-generation-in-assemblers).

---

GCC's initial initial RISC-V port made a mistake by having both `call
foo` (non-PIC) and `call foo@plt` (PIC), likely misled by x86/SystemZ.
It was determined that the `@plt` was not needed. Since R_RISCV_CALL had
questionable undefined weak semantics in GNU ld (which has been removed
then), we kept R_RISCV_CALL_PLT and deprecated R_RISCV_CALL.

For RISC-V instructions, we only keep `@` in call/jump for backward
compatibility and discourage it for all other instructions.

(
There is disagreement about whether `PLT` in `JUMP_PLT` is useful or
misleading.
MaskRay's opnion: For new branch relocations with procedure call
semantics, use `_CALL` and avoid `_PLT` in the relocation name.
`_PLT` should only be used in data directives (e.g. R_RISCV_PLT32) to
indicate that the address of a function is not significant.
)

Pull Request: https://github.com/llvm/llvm-project/pull/135507

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