qt: remove Kconfig option to enable all pixel depths

The Kconfig menu "Pixel depths" of Qt provides a list with all
selectable pixel depths and additionally the option "all".

When "all" is selected, the list with all available pixel depths
disappears.

Because this disappearing of available pixel depths makes no sense
under usability aspects, simply remove the option "all".

The user could enable all available pixel depths by selecting each
individual depth anyway. So no functionality is lost.

Signed-off-by: Michael Roth <mroth@nessie.de>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>