do not use large buffers if not necessary (#11220)

without this change, there is a performance
regression for small objects GETs, this makes
the overall speed to go back to pre '59d363'
commit days.

cmd/storage-rest-common.go

@@ -59,7 +59,6 @@ const (
 	storageRESTDirPath        = "dir-path"
 	storageRESTFilePath       = "file-path"
 	storageRESTVersionID      = "version-id"
-	storageRESTReadData       = "read-data"
 	storageRESTTotalVersions  = "total-versions"
 	storageRESTSrcVolume      = "source-volume"
 	storageRESTSrcPath        = "source-path"