Remove deprecated USE_GNOME2_MACROS from autogen.sh

Use of USE_GNOME2_MACROS is deprecated in GNOME 3 and produced this
warning:

    $ ./autogen.sh
    /usr/bin/gnome-autogen.sh
    ...
    ***Warning*** USE_GNOME2_MACROS is deprecated, you may remove it from autogen.sh
    ...

It's use appears to have been removed first from GNOME 2.8 with this
commit from 2004:

    https://git.gnome.org/browse/gnome-common/commit/?id=ea9e85851445efa0135c3f8d08c3d1ea53760d91
    delete some files that were unused after the reorganisation

The oldest supported distribution is RHEL / CentOS 6 which is using
gnome-common-2.28.0 from 2009.  Therefore unconditionally remove the
USE_GNOME2_MACROS setting.

Also confirmed that it makes no difference by running ./autogen.sh with
and without USE_GNOME2_MACROS being set.  The produced GParted build
trees were the same.  Therefore the release and executable can't be
affected.