docs-xml: remove explicit "constant"

docs-xml: remove explicit "constant"

The constant mark applied to types "string" and "ustring". The previous patches
in this patchset already markes all string options as either constant or
substituted, but it's still possible to add options or change existing ones to
be neither constant nor substituted.

In order to enforce strings to be either constant or substitued, remove the
explicit constant marker. Instead, any option that is not marked as substituted
is implicitly made constant.

This patch doesn't change behaviour and all generated files are the same before
and after this change.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>