desktop: Remove obsolete Bugzilla entries

The `X-GNOME-Bugzilla-*` entries were for use by bug-buddy, a GNOME 2
technology that's been gone for over a decade. These entries are
obsolete and can be removed from all desktop files.

The `X-GNOME-Settings-Panel` entry is also obsolete as far as I can
tell and only these panels had it in their desktop file: notifications,
sharing, sound and user-accounts. These entries can also be removed.

After removing the `X-GNOME-Bugzilla-*` entries, the desktop files have
no more variables in them. The meson `configure_file` step is therefor
pointless—there are no variables to configure. As such the
`*.desktop.in.in` files are renamed to `*.desktop.in` to reflect this
and `meson.build` files are modified to remove `configure_file` step.
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