dhcp/router_delete_namespaces[1] options have been defined as a
workaround to an iproute2 limitation[1] corrected 2 years ago.
That's why the change removes these options after their deprecation
in Liberty.
[1] in neutron.agent.dhcp/l3.config
DocImpact
Closes-Bug: #1508189
Related-Bug: #1418079
Change-Id: I2a879213c3b095a007a4531f430a33cea9fdf1bd