Change: Removed filter button in files list header

Removed the confusing filter button in the files list header, so the following are now removed as well:
- ability to toggle files and folders visibility which wasn't that useful and not really a requirement
- filter text box as it is is redundant as one can already use the global search box
- ability to hide dot files, we'll look into providing this again in the future with an improved UI

https://github.com/owncloud/web/issues/2971
