network: rename link_drop_managed_xyz() -> link_drop_static_xyz()

network: rename link_drop_managed_xyz() -> link_drop_static_xyz()

As networkd does not configure nexthops, neighbors, or routing policy rules
dynamically, this only changes link_drop_managed_addresses(). Previously
it also removed dynamic addresses, but with this commit, now it only
removes static addresses. Note, link_drop_managed_routes() has removed
only static routes, so this commit only changes its name.

Note. as commented in the code, the dynamic addresses will be removed
on lease expiration or stopping client, and link_drop_managed_xyz() are
called on reconfiguration or so, and at that time, all dynamic
addressing clients are stopped or restarted, so this should not
effective behavior change.