um: time-travel: remove time_exit()

This function is unused and unneeded, remove it.

Link: https://patch.msgid.link/20240703130105.02b3a974acb7.I7264821f7cfa17ea713b7a3e4787aa41a3107d01@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>

arch/um/kernel/time.c

@@ -1038,11 +1038,5 @@ static int __init um_bc_start(void)
 
 	return 0;
 }
-
-void __exit time_exit(void)
-{
-	kobject_put(bc_time_kobject);
-}
-
 late_initcall(um_bc_start);
 #endif