Hi,

This patch adds a 'none setting for org-agenda-overriding-header, which
allows it to be disabled completely, rather than inserting a blank line
as it does when set to an empty string.

This was requested by a user here:
<https://github.com/alphapapa/org-super-agenda/issues/10>, but I guess
it may also be useful for Org users who don't use that package.

Thanks,
Adam