selinux: drop return at end of void function avc_insert()

Commit 539813e4

 ("selinux: stop returning node from avc_insert()")
converted the return value of avc_insert() to void but left the now
unnecessary trailing return statement.

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>