build: drop vestigial 'supported Qt distro' check

This check was added in 5ec47bc3 about
two years ago when client-qt was just beginning to be ported to
GNU/Linux and could not be readily/easily built on GNU/Linux systems.

This removal should also fix/avoid the recently-introduced logic bug
that broke --distribution=android.

Change-Id: Ia9ab9e37b9d9cfd2f7a2a57931bbd5bf6f4d56bb