Replace deleteAll call with unlink call

The method deleteAll() doesn't officially exist on the Storage class as
it's not defined on the interface, which means it fails on the Quota
storage wrapper and might fail on some external storage classes.
Also, this here is the only use case for that one method.

lib/private/files/view.php

@@ -413,7 +413,7 @@ class View {
 						$result = $this->copy($path1, $path2);
 						if ($result === true) {
 							list($storage1, $internalPath1) = Filesystem::resolvePath($absolutePath1 . $postFix1);
-							$result = $storage1->deleteAll($internalPath1);
+							$result = $storage1->unlink($internalPath1);
 						}
 					} else {
 						$source = $this->fopen($path1 . $postFix1, 'r');