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