Remove `riscv` feature flag #12417

Job	Run time
check-dependency-rules	10s
check-fail-ci	12s
check-workspace	24s
check-markdown	15s
preflight / preflight	46s
preflight / ci-versions	1m 0s
check-rust-feature-propagation	2m 6s
check-toml-format	1m 1s
fmt	1m 14s
test-rust-features	2m 5s
check-umbrella	2m 23s
All quick checks passed	1s
	11m 37s

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