[llvm] [RISCV] Xqcilb: remove RISCVMCExpr::VK_QC_E_JUMP_PLT and drop `@plt` parsing (PR #135507)

[Sam Elliott via llvm-commits]

lenary wrote:

The encodings are in the Xqci releases here (there's a pdf uploaded to each release, but this repo contains all Xqc* architecture descriptions): https://github.com/quic/riscv-unified-db/releases

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