[fix] get rid of checking for absent device name in vfs_get_tree()

It has no business being there, it's checked by relevant ->get_tree()
as it is *and* it returns the wrong error for no reason whatsoever.

Fixes: f3a09c92 "introduce fs_context methods"
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/super.c

@@ -1467,11 +1467,6 @@ int vfs_get_tree(struct fs_context *fc)
 	struct super_block *sb;
 	int error;
 
-	if (fc->fs_type->fs_flags & FS_REQUIRES_DEV && !fc->source) {
-		errorf(fc, "Filesystem requires source device");
-		return -ENOENT;
-	}
-
 	if (fc->root)
 		return -EBUSY;