Docbook build: remove MAKE fallback

The script has been running for over 10 years calling Publican directly
(see Issue #15887).

A fallback using the legacy MAKE method was kept for backwards-
compatibility when publican.cfg is not present, but this part of the
script is not functional anymore; even if it were, it would not work
anyway since the Makefile calls Publican, which would fail due to the
missing config file.

Fixes 0037305