Revert "integrity: double check iint_cache was initialized"

With the recent introduction of LSM_ORDER_LAST, the 'integrity' LSM is
always initialized (if selected in the kernel configuration) and the
iint_cache is always created (the kernel panics on error). Thus, the
additional check of iint_cache in integrity_inode_get() is no longer
necessary. If the 'integrity' LSM is not selected in the kernel
configuration, integrity_inode_get() just returns NULL.

This reverts commit 92063f3c.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Acked-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>