docs: remove unnecessary `CofactorGroup` documentation #424

Job	Run time
Check that benchmarks compile	3m 1s
Build serial on no_std target (thumbv7em-none-eabi) (curve25519-dalek)	19s
Check that clippy is happy	24s
Build serial on no_std target (thumbv7em-none-eabi) (ed25519-dalek)	28s
Build serial on no_std target (thumbv7em-none-eabi) (x25519-dalek)	23s
Check docs	40s
Check formatting	8s
Test Nightly	1m 10s
Test 32/64 bit stable (i686-unknown-linux-gnu, sudo apt update && sudo apt install gcc-multilib)	2m 46s
Test 32/64 bit stable (x86_64-unknown-linux-gnu)	2m 4s
	11m 23s

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