simplify rpc codec logic #18300

Job	Run time
check-msrv	5m 2s
Check for 'skip-ci' label	1s
check-code	9m 50s
target-branch-check	1s
beacon-chain-tests	16m 7s
cargo-udeps	7m 36s
cli-check	7m 58s
basic-simulator-ubuntu	13m 20s
release-tests-windows	32m 48s
compile-with-beta-compiler	13m 49s
debug-tests-ubuntu	10m 7s
ef-tests-ubuntu	13m 37s
execution-engine-integration-ubuntu	13m 42s
fallback-simulator-ubuntu	10m 53s
network-tests	8m 46s
op-pool-tests	12m 13s
release-tests-ubuntu	17m 27s
slasher-tests	1m 43s
state-transition-vectors-ubuntu	2m 36s
test-suite-success	5s
	3h 17m 41s

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