qt4: Avoid duplicate flags in the g++-unix.conf

The removed flags from this g++.conf file, which is installed to the
target as g++-unix.conf are added by gcc-base.conf . Do not add those
flags twice.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Eric Bénard <eric@eukrea.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>