Clean up includes of s_lock.h.

Users of spinlocks should use spin.h, not s_lock.h. And lwlock.h
hasn't utilized spinlocks for quite a while.

Discussion: https://postgr.es/m/20200618183041.upyrd25eosecyf3x@alap3.anarazel.de

src/backend/main/main.c

@@ -35,7 +35,6 @@
 #include "common/username.h"
 #include "port/atomics.h"
 #include "postmaster/postmaster.h"
-#include "storage/s_lock.h"
 #include "storage/spin.h"
 #include "tcop/tcopprot.h"
 #include "utils/help_config.h"

src/include/storage/condition_variable.h

@@ -23,7 +23,7 @@
 #define CONDITION_VARIABLE_H
 
 #include "storage/proclist_types.h"
-#include "storage/s_lock.h"
+#include "storage/spin.h"
 
 typedef struct
 {

src/include/storage/lwlock.h

@@ -20,7 +20,6 @@
 
 #include "port/atomics.h"
 #include "storage/proclist_types.h"
-#include "storage/s_lock.h"
 
 struct PGPROC;