Removes services.hyperhive.deploy.matrix.gui.enable and its
swarm.matrix.gui.enable alias; both are mkRemovedOptionModule stubs. A
host running the homeserver serves fluffychat at gatewayHost's vhost,
and the hive's /matrix/ redirect follows the same condition.
The swarm-controller builds the Matrix quick link from
swarm.matrix.gatewayHost, replacing hive-matrix.nix's per-host entry.
HIVE_MATRIX_PUBLIC_URL is set on every hive with a gatewayHost, so
`hivectl open matrix` resolves off the homeserver's host too.
Drops HIVE_MATRIX_GUI_ENABLED and the dashboard's matrix_gui_enabled
field; nothing in the frontend reads it.
Refs #4885
f60f8af33b