security/integrity: remove unnecessary 'init_keyring' variable

The 'init_keyring' variable actually just gave the value of
CONFIG_INTEGRITY_TRUSTED_KEYRING.  We should check the config option
directly instead.  No change in behavior; this just simplifies the code.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>