[filesapp] Remove chrome.mediaGalleries from UITest system

Files app removed use of chrome.mediaGalleries a few milestones ago so
this code is no longer used. Remove it.

Bug: code-health
Change-Id: I38fea9943fe8e01530d79e2137ba498f2047c8b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2526824
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Auto-Submit: Noel Gordon <noel@chromium.org>
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825935}

ui/file_manager/file_manager/test/js/chrome_file_manager_private_test_impl.js

@@ -295,21 +295,6 @@ chrome.fileManagerPrivate = {
   },
 };
 
-/**
- * Suppress compiler warning for overwriting chrome.mediaGalleries.
- * @suppress {checkTypes}
- */
-chrome.mediaGalleries = {
-  getMetadata: (mediaFile, options, callback) => {
-    // Returns metdata {mimeType: ..., ...}.
-    setTimeout(() => {
-      webkitResolveLocalFileSystemURL(mediaFile.name, entry => {
-        callback({mimeType: entry.metadata.contentMimeType});
-      }, 0);
-    });
-  },
-};
-
 /**
  * Suppress compiler warning for overwriting chrome.fileSystem.
  * @suppress {checkTypes}