backup: always write to cloud storage in processor

This removes the bulkio.backup.proxy_file_writes.enabled setting, and just
always does the behavior of it being true.

Release note (sql change): the setting bulkio.backup.proxy_file_writes.enabled is no longer needed to enable proxied writes which are now the default.

pkg/ccl/backupccl/backup_processor.go

@@ -68,18 +68,6 @@ var (
 		time.Minute*5,
 		settings.NonNegativeDuration,
 	)
-	alwaysWriteInProc = settings.RegisterBoolSetting(
-		"bulkio.backup.proxy_file_writes.enabled",
-		"return files to the backup coordination processes to write to "+
-			"external storage instead of writing them directly from the storage layer",
-		false,
-	)
-	smallFileSize = settings.RegisterByteSizeSetting(
-		"bulkio.backup.merge_file_size",
-		"size under which backup files will be forwarded to another node to be merged with other smaller files "+
-			"(and implies files will be buffered in-memory until this size before being written to backup storage)",
-		16<<20,
-		settings.NonNegativeInt,
 	targetFileSize = settings.RegisterByteSizeSetting(
 		"bulkio.backup.file_size",
 		"target file size",
@@ -230,8 +218,6 @@ func runBackupProcessor(
 		storageConfByLocalityKV[kv] = &conf
 	}
 
-	// If this is a tenant backup, we need to write the file from the SQL layer.
-	writeSSTsInProcessor := !flowCtx.Cfg.Codec.ForSystemTenant() || alwaysWriteInProc.Get(&clusterSettings.SV)
 
 	returnedSSTs := make(chan returnedSST, 1)
 
@@ -268,14 +254,8 @@ func runBackupProcessor(
 						StartTime:                           span.start,
 						EnableTimeBoundIteratorOptimization: useTBI.Get(&clusterSettings.SV),
 						MVCCFilter:                          spec.MVCCFilter,
-						ReturnSstBelowSize:                  smallFileSize.Get(&clusterSettings.SV),
 						TargetFileSize:                      storageccl.ExportRequestTargetFileSize.Get(&clusterSettings.SV),
-					}
-					if writeSSTsInProcessor {
-						req.ReturnSST = true
-					} else {
-						req.Storage = defaultConf
-						req.Encryption = spec.Encryption
+						ReturnSST:                           true,
 					}
 
 					// If we're doing re-attempts but are not yet in the priority regime,

pkg/settings/registry.go

@@ -101,6 +101,7 @@ var retiredSettings = map[string]struct{}{
 	"kv.tenant_rate_limiter.write_cost_per_megabyte":                 {},
 	"kv.transaction.write_pipelining_max_outstanding_size":           {},
 	"sql.defaults.optimizer_improve_disjunction_selectivity.enabled": {},
+	"bulkio.backup.proxy_file_writes.enabled":                        {},
 }
 
 // register adds a setting to the registry.