xbps-src.conf: XBPS_DESTDIR option removed

[Juan RP]

Hi,

I removed XBPS_DESTDIR option from xbps-src.conf. Now it's always
defined as $XBPS_MASTERDIR/pkg-destdir, so that you don't have to
care about its location. I believe that making this configurable
wasn't so useful after all.

To upgrade to new format, reinstall xbps-src in host and chroot,
update your xbps-src.conf file and move your old XBPS_DESTDIR
to $XBPS_MASTERDIR/pkg-destdir.

Regards.