textproc/docproj: Remove legacy dependencies

Clean the current port for people just building the new documentation
with Hugo/Asciidoctor.

Keep the old dependencies list on a new port textproc/docproj-legacy.

Reviewed by: hrs
Approved by: doceng (with hat)
Differential Revision: https://reviews.freebsd.org/D31019

(cherry picked from commit ffdb67aee0747f0465be6a2f0f897ddfac8ecaee)