Remove set_latch_on_sigusr1 flag.

This flag has proven to be a recipe for bugs, and it doesn't seem like
it can really buy anything in terms of performance.  So let's just
*always* set the process latch when we receive SIGUSR1 instead of
trying to do it only when needed.

Per my recent proposal on pgsql-hackers.

src/include/storage/procsignal.h

@@ -55,6 +55,5 @@ extern int SendProcSignal(pid_t pid, ProcSignalReason reason,
 			   BackendId backendId);
 
 extern void procsignal_sigusr1_handler(SIGNAL_ARGS);
-extern PGDLLIMPORT bool set_latch_on_sigusr1;
 
 #endif   /* PROCSIGNAL_H */