Revert "geom_bde: plug set-but-not-used vars"

The commit at hand happens to break userspace build as the header ins
included by sbin/gbde/gbde.c and the __diagused macro is not provided to
userspace.

Revert until this gets sorted out.

This reverts commit 26e837e2d488cc3e484c88a511728e061205edc5.