Documentation: remove current_security() reference

Commit 15322a0d ("lsm: remove current_security()") removed
current_security() from the sources.
Signed-off-by: Denis Efremov <efremov@linux.com>
Link: https://lore.kernel.org/r/20200830142509.5738-1-efremov@linux.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/security/credentials.rst

@@ -323,7 +323,6 @@ credentials (the value is simply returned in each case)::
 	uid_t current_fsuid(void)	Current's file access UID
 	gid_t current_fsgid(void)	Current's file access GID
 	kernel_cap_t current_cap(void)	Current's effective capabilities
-	void *current_security(void)	Current's LSM security pointer
 	struct user_struct *current_user(void)  Current's user account
 
 There are also convenience wrappers for retrieving specific associated pairs of