webadmin: Clean up Common.gwt.xml

Cleaned up Common.gwt.xml from numerous entries in order to make the
file more maintainable and speed up the build a tad.

The following entries were removed:
1. Classes that no longer exist in the code base (e.g.,
   time_lease_vm_pool_map).
2. Classes that are not used by the frontend (e.g.,
   LUN_storage_server_connection_map).
3. Double entries (e.g., UsbPolicy).
4. Specific entries that can be inferred from more general entries
   (e.g., common/action/VdcActionType.java does not need to be specified
   since common/action/* already is).

Change-Id: Idebce672b96192f7b114fc1596ba462fe097d9b8
Signed-off-by: Allon Mureinik <amureini@redhat.com>