It was deprecated at [1].
Remove the deprecated config 'router_id' and its related tests.
[1] https://review.openstack.org/#/c/248498
DocImpact: All references of 'router_id' configuration option
and its description should be removed from the docs.
UpgradeImpact: Remove 'router_id' configuration option from the
l3_agent.ini.
Change-Id: Ic9420191e8c1a333e4dcc0b70411591b8573ec7c
Closes-Bug: #1594711