do not force a specific export-dir in the repository

Git-Dch: Ignore

debian/gbp.conf

@@ -1,6 +1,5 @@
 [DEFAULT]
 prebuild = ./prepare-release pre-export
 postbuild = ./prepare-release post-build
-export-dir = ../build-area
 debian-branch = debian/sid
 debian-tag = %(version)s