Removes available_providers filter in ProvidersController

This:

a.) Served no constructive purpose. It was ostensibly to allow
 upstream to support everything, but product to only support
 fully-tested providers. Except the file was never changed.

b.) Does not belong in a controller.

c.) Breaks if you send a patch like my 'Openstack' -> 'OpenStack'
 one, unless I were to check for both strings, which would be
 a ridiculous mess.