aead: remove `AeadCore::CiphertextOverhead` (#1737) #2986

Job	Run time
build (1.83.0, thumbv7em-none-eabi)	1m 10s
minimal-versions	0s
build (1.83.0, wasm32-unknown-unknown)	1m 2s
build (stable, thumbv7em-none-eabi)	1m 3s
test (1.83.0)	33s
build (stable, wasm32-unknown-unknown)	57s
test (stable)	30s
test (nightly)	34s
	5m 49s

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