qt: remove redundant and incorrect URL from Makefile

The upstream URL for Buildroot packages is usually visible in the Config.in
file, not in comments in the Makefile.

Moreover, this URL does not point to the correct upstream project.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>