kernel/pid.c: remove unneeded hash header file

Hash functions are not needed since idr is used now.  Let's remove hash
header file for cleanup.

Link: http://lkml.kernel.org/r/20190430053319.95913-1-scuttimmy@gmail.comSigned-off-by: Timmy Li <scuttimmy@gmail.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: KJ Tsanaktsidis <ktsanaktsidis@zendesk.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/pid.c

@@ -32,7 +32,6 @@
 #include <linux/init.h>
 #include <linux/rculist.h>
 #include <linux/memblock.h>
-#include <linux/hash.h>
 #include <linux/pid_namespace.h>
 #include <linux/init_task.h>
 #include <linux/syscalls.h>