flatpak: Stop adding the "gnome" remote
Over on sdk.gnome.org, there are two remotes providing GNOME Platform and Sdk runtimes:
- gnome
- gnome-nightly
While the latter is still very much in use, the former hasn't received any update for about a year. The last build was for GNOME 3.28 in May 2018.
Stable GNOME runtimes are now hosted on Flathub instead.
As a result, there is no point for Builder to configure this remote.
See also: #555 (comment 262899)