camera: Remove portal usage

At the moment there are multiple bugs that make this not worth the effort:

• https://github.com/flatpak/xdg-dbus-proxy/issues/46: Sometimes the portal will just fail for no good reason making us fallback to the non-sandboxed method.
• When opening the app for the first time the dialog that asks for camera permissions is not shown, forcing a big number of users to use Flatseal. Note that we request the portal 250ms after the window is mapped. See gnome-shell#7548.
• When the dialog fails to be shown it is assumed that the user denied the permission
• The permission dialog automatically denies the permission after a timeout #155 (closed)
• It is not possible for the app to request permission. All of the previous 4 items together mean that once the app is open the user is banned from user the camera.
• GNOME Settings cannot toggle the permissions correctly gnome-control-center#2993 (closed) and gnome-control-center#2982 (closed).
• Camera portal behaves differently when the app is started from cli than when started from the app overview. Seems to only affect the unsandboxed version #154 and https://bugzilla.redhat.com/show_bug.cgi?id=2274061.

We have been not able to reproduce the second and third error.

Fixes: #136 (closed)

cc @mcatanzaro