Remove legacy routing code

The getfile routing code was absolutely legacy and not needed anymore. Additionally \OC::$REQUESTEDAPP was never set to the actually accessed application.

This commit removes the legacy routing code and ensures that $REQUESTEDAPP is always set so that other applications (e.g. the firewall or a two-factor authentication) can intercept the currently accessed app.

Testplan:
- [x] Installation works
- [x] Login with DB works
- [x] Logout works
- [x] Login with alternate backend works (tested with user_webdavauth)
- [x] Other apps are accessible
- [x] Redirect on login works (e.g. index.php?redirect_url=%2Fcore%2Findex.php%2Fsettings%2Fapps%3Finstalled)
- [x] Personal settings are accessible
- [x] Admin settings are accessible
- [x] Sharing files works
- [x] DAV works
- [x] OC::$REQUESTEDAPP contains the requested application and can be intercepted by other applications

@karlitschek @PVince81 Please review. This addresses https://github.com/owncloud/security-tracker/issues/70