keys: Remove unused extern declarations

Since commit b2a4df20 ("KEYS: Expand the capacity of a keyring")
iterate_over_keyring() is never used, so can be removed.

And commit b5f545c8 ("[PATCH] keys: Permit running process to instantiate keys")
left behind keyring_search_instkey().

Fixes: b2a4df20 ("KEYS: Expand the capacity of a keyring")
Fixes: b5f545c8

 ("[PATCH] keys: Permit running process to instantiate keys")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>