gs-prefs-dialog: Remove download-updates switch from the preferences

Since automatic updates are now controlled by Mogwai, which looks at
preferences per network connection, this setting is no longer used and
should no longer be presented in the UI.

There’s an argument for reworking the entire preferences dialogue now,
since it now only contains a single switch; but that can wait for a
later commit.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
1 job from metered-data2 in 4 minutes and 48 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
failed #247096
build-gnome-software

00:04:48

 
Name Stage Failure
failed
build-gnome-software Build
Dependency json-glib-1.0 found: YES 1.4.4
Library m found: YES
Dependency libsoup-2.4 found: YES 2.64.2
Dependency mogwai-schedule-client-0 found: NO (tried pkgconfig)

meson.build:109:0: ERROR: Dependency "mogwai-schedule-client-0" not found, tried pkgconfig

A full log can be found at /builds/pwithnall/gnome-software/_build/meson-logs/meson-log.txt
ERROR: Job failed: exit code 1