Remove redundant SetProcessingMode(InitProcessing) calls

After several refactoring iterations, auxiliary processes are no
longer initialized from the bootstrapper. Using the InitProcessing
mode for initializing auxiliary processes is more appropriate. Since
the global variable Mode is initialized to InitProcessing, we can just
remove the redundant calls of SetProcessingMode(InitProcessing).

Author: Xing Guo <higuoxing@gmail.com>
Discussion: https://www.postgresql.org/message-id/CACpMh%2BDBHVT4xPGimzvex%3DwMdMLQEu9PYhT%2BkwwD2x2nu9dU_Q%40mail.gmail.com

src/backend/postmaster/autovacuum.c

@@ -380,7 +380,7 @@ AutoVacLauncherMain(char *startup_data, size_t startup_data_len)
 	if (PostAuthDelay)
 		pg_usleep(PostAuthDelay * 1000000L);
 
-	SetProcessingMode(InitProcessing);
+	Assert(GetProcessingMode() == InitProcessing);
 
 	/*
 	 * Set up signal handlers.  We operate on databases much like a regular
@@ -1373,7 +1373,7 @@ AutoVacWorkerMain(char *startup_data, size_t startup_data_len)
 	MyBackendType = B_AUTOVAC_WORKER;
 	init_ps_display(NULL);
 
-	SetProcessingMode(InitProcessing);
+	Assert(GetProcessingMode() == InitProcessing);
 
 	/*
 	 * Set up signal handlers.  We operate on databases much like a regular

src/backend/postmaster/auxprocess.c

@@ -54,7 +54,8 @@ AuxiliaryProcessMainCommon(void)
 
 	init_ps_display(NULL);
 
-	SetProcessingMode(BootstrapProcessing);
+	Assert(GetProcessingMode() == InitProcessing);
+
 	IgnoreSystemIndexes = true;
 
 	/*

src/backend/postmaster/bgworker.c

@@ -746,7 +746,7 @@ BackgroundWorkerMain(char *startup_data, size_t startup_data_len)
 	MyBackendType = B_BG_WORKER;
 	init_ps_display(worker->bgw_name);
 
-	SetProcessingMode(InitProcessing);
+	Assert(GetProcessingMode() == InitProcessing);
 
 	/* Apply PostAuthDelay */
 	if (PostAuthDelay > 0)

src/backend/replication/logical/slotsync.c

@@ -1342,7 +1342,7 @@ ReplSlotSyncWorkerMain(char *startup_data, size_t startup_data_len)
 
 	init_ps_display(NULL);
 
-	SetProcessingMode(InitProcessing);
+	Assert(GetProcessingMode() == InitProcessing);
 
 	/*
 	 * Create a per-backend PGPROC struct in shared memory.  We must do this

src/backend/tcop/postgres.c

@@ -4161,7 +4161,7 @@ PostgresMain(const char *dbname, const char *username)
 	Assert(dbname != NULL);
 	Assert(username != NULL);
 
-	SetProcessingMode(InitProcessing);
+	Assert(GetProcessingMode() == InitProcessing);
 
 	/*
 	 * Set up signal handlers.  (InitPostmasterChild or InitStandaloneProcess