cgroup: remove gratuituous BUG_ON()s from rebind_subsystems()

rebind_subsystems() performs santiy checks even on subsystems which
aren't specified to be added or removed and the checks aren't all that
useful given that these are in a very cold path while the violations
they check would trip up in much hotter paths.

Let's remove these from rebind_subsystems().

Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup.c

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index a65aff1..864a4b2 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1071,15 +1071,6 @@
 			/* subsystem is now free - drop reference on module */
 			module_put(ss->module);
 			root->subsys_mask &= ~bit;
-		} else if (bit & root->subsys_mask) {
-			/* Subsystem state should already exist */
-			BUG_ON(!cgrp->subsys[i]);
-#ifdef CONFIG_MODULE_UNLOAD
-			BUG_ON(ss->module && !module_refcount(ss->module));
-#endif
-		} else {
-			/* Subsystem state shouldn't exist */
-			BUG_ON(cgrp->subsys[i]);
 		}
 	}