x509-cert: remove unnecessary `builder` feature activations #2303

Job	Run time
build (1.81.0, thumbv7em-none-eabi)	59s
test (1.81.0)	1m 22s
build (1.81.0, wasm32-unknown-unknown)	55s
build (stable, wasm32-unknown-unknown)	45s
build (stable, thumbv7em-none-eabi)	54s
test (stable)	1m 6s
minimal-versions	0s
fuzz	2m 10s
	8m 11s

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