Clean up comments in FileSystem

Problem/Motivation

core/lib/Drupal/Core/File/FileSystem.php contains lots of comments that are not necessary.

Proposed resolution

Remove unnecessary comments. See inline commenting standards https://www.drupal.org/docs/develop/standards/api-documentation-and-comm...

Remaining tasks

Do it.

User interface changes

None.

Original summary

Follow up to #2244513: Move the unmanaged file APIs to the file_system service (file.inc) There were a number of comments that were copy/pasted from the original.

+++ b/core/lib/Drupal/Core/File/FileSystem.php
@@ -301,4 +307,331 @@ public function validScheme($scheme) {
+    // Assert that the source file actually exists.
+    if (!file_exists($source)) {
...
+    // Check if directory exists.
+    if (!is_dir($directory)) {

I think we should remove comments like this - they add no value.

Let's add a follow-up to check all the comments in this file because I these are copy pastes.