Remove experimental permission.

This should no longer be required, causes a warning on chrome://extensions and may prevent CWS upload.

Review URL: https://codereview.chromium.org/1088213004

Cr-Commit-Position: refs/heads/master@{#326908}

remoting/webapp/app_remoting/manifest_common.json.jinja2

@@ -27,7 +27,6 @@
     "clipboardRead",
     "clipboardWrite",
     "contextMenus",
-    "experimental",
     "fileSystem",
     "fullscreen",
     "https://relay.google.com/*",