[llvm] [RISCV] Remove redundant FeatureStdExtZcd from X100. (PR #178928)

[Yingwei Zheng via llvm-commits]

https://github.com/dtcxzyw approved this pull request.

LG

Corresponding rule: https://github.com/riscv/riscv-isa-manual/blob/f2dd81cfa187d57d5b24697038ea9ebe2adc7874/src/zc.adoc?plain=1#L64

https://github.com/llvm/llvm-project/pull/178928