gbp: do not set export-dir

When attempting to build the package on a labs machine I ended up with
the following error:

 gbp:error: Cannot create output dir
 /home/jenkins-deploy/build-area/kafka

Turns out there is no build-area there and I dont even want to use the
home dir as a build area :-)  I guess one would want to use its personal
configuration file for that: ~/.gbp.conf.

Change-Id: I0670e1bb97a31c9e230fc15b273288e56835d528