Remove dead method _get_router_ids

Remove dead method _get_router_ids

No place is using _get_router_ids, it is dead method now.
And for l3_db.Router related query, it always based on certain
filters, such as on which host, extra attribute is distributed
or HA. While this method cannot fix those requirements.

Method _get_router_ids is created in patch
I35eae975209316aad6b2c97c909dce385729864d , its purpose is to
avoid DB query to fetch full data for routers while only id is
needed. But in patch Ie561b19a2e58a2a563d79b75421e9e24c70f36f9 ,
its caller was removed, but seems we forget to remove this dead
method at that time.

Change-Id: I924e3e652deea2f89c4aaf1fd780e467ea81b843