Revert "MDL-40931 useragent: separated user agent functionality into a lib"

This reverts commit af62237d0e.
This reverts commit 87a4194fdc.
This reverts commit b2c66eb60c.

Conflicts:

	version.php

theme/switchdevice.php

@@ -31,6 +31,6 @@ $newdevice = required_param('device', PARAM_TEXT);
 
 require_sesskey();
 
-core_useragent::set_user_device_type($newdevice);
+set_user_device_type($newdevice);
 
 redirect($url);