qt: remove unnecessary call to qstrip macro in Makefile

QT_CONFIGURE can never gain double quotes, so there
is no point to remove some.

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>