Do not run pre*export* functions in the pre*build* hook

Unfortunately it seems like git-buildpackage does not have a
pre-export hook so the hook is disabled for now.

Git-Dch: ignore

debian/gbp.conf

@@ -1,5 +1,6 @@
 [DEFAULT]
-prebuild = ./prepare-release pre-export
+; we need preexport here
+;prebuild = ./prepare-release pre-export
 postbuild = ./prepare-release post-build
 debian-branch = debian/experimental
 debian-tag = %(version)s