Remove `riscv` feature flag #4621

Job	Run time
preflight / preflight	32s
preflight / ci-versions	1m 3s
test-full-crypto-feature	2m 57s
cargo-check-benches (master)	20m 56s
cargo-check-benches (current)	24m 5s
test-frame-ui	46m 7s
test-node-metrics	9m 40s
cargo-check-all-crate-macos	11m 21s
cargo-check-each-crate (1)	22m 38s
cargo-check-each-crate (2)	20m 34s
cargo-check-each-crate (3)	9m 23s
cargo-check-each-crate (4)	20m 18s
cargo-check-each-crate (5)	27m 48s
cargo-check-each-crate (6)	14m 55s
cargo-check-each-crate (7)	31m 5s
test-frame-examples-compile-to-wasm	3m 8s
test-deterministic-wasm	7m 57s
check-tracing	2m 32s
check-metadata-hash	7m 4s
node-bench-regression-guard	14s
All test misc tests passed	1s
	4h 44m 18s

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