Remove duplicated check for router connect to external net

In the function get_assoc_data, we check the router which get
from _get_router_for_floatingip whether connect to external net,
but as it has already checked in _get_router_for_floatingip.

Change-Id: I0c00c480273145538d19569dc0a679e9935ad8f9
Closes-bug: #1353414
Merge "Remove duplicated check for router connect to external net"