remove code in dead HTML_BEHAVIOUR #define

git grep found nowhere in the codebase that this define is being
set.

Change-Id: I63e18e8f47b5e58ff96b075ddb4bc9a36e1dc80e
Reviewed-on: https://gerrit.libreoffice.org/8002
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>