feature/conn25,ipn/ipnlocal: remove UseWIPCode() guards

Removing this check installs conn25 instance-level hooks whenever the
feature is built in. init-level hooks were always installed, but used
this guard to exit early.

Now all hooks, both instance-level and init-level rely on netmap
configuration (populated tailscale.com/app-connectors-experimental node
attribute, with non-empty apps) to not exit early.

The conn25-shutoff feature flag tells control not to send that node
attribute.

Fixes tailscale/corp#39033

Signed-off-by: Michael Ben-Ami <mzb@tailscale.com>