shell: Don't use the OnlyShowIn/NotShowIn flags from desktop files to hide panels

It was added in 3e04b1c5 to allow downstreams to hide panels that don't make
sense in their environment. Later on the ability for panels to hide themselves
was added in a78cbe39, which is a better solution for this case.

The current situation is confusing if you are not a known environment, as then no
panels show.

Fixes #163
Fixes Librem5/gnome-control-center#45
4 jobs for wip/purism/fix-ssh-crash in 16 minutes and 32 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Package
passed #249085
librem5
package:deb-debian-buster

00:04:52

passed #249086
librem5:arm64
package:deb-debian-buster:arm64

00:10:32

 
  Test Package
passed #249087
librem5
autopkgtest-debian-buster-package

00:01:00

passed #249088
librem5
lintian-debian-buster-package

00:01:07