Removes the old UI build tooling; it is no longer necessary
because as of 1.2.0 it's bundled into the server binary.
It doesn't even need to have JS built, because it's bundled into
the release commit's source tree (see #48714).
The UI is enabled by default, so the NixOS service is
updated to directly use `ui = webUi;` now.
Fixes#48714.
Fixes#44192.
Fixes#41243.
Fixes#35602.
Signed-off-by: Niklas Hambüchen <mail@nh2.me>
2cb7f5fb1e