qt: remove PKG_CONFIG workaround

At least Qt 4.5.2 is fixed in the meantime and honors PKG_CONFIG.
So the PATH workaround is no longer needed.

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>