Revert "Support --with=dbgonly for debug pipelines"

This reverts commit 0a4f8c4c.

--with=dbgonly has been superseeded by --without=base [1][2], which makes it possible to handle it with the usual variant logic.

Fixes #205 (closed)

[1] kernel-ark!2719 (merged)
[2] !1836 (merged)

Signed-off-by: Michael Hofmann mhofmann@redhat.com