Removed dead code for gradle-docbook-plugin

When I first started looking for a gradle plugin
wrapper around DocBook, I found gradle-docbook-plugin.

Initial attempts proved futile in getting it to work
but then I saw that in 0d84460, it looks like the use of
it was finally deprecated (without an explicit reason, though it
looks like it was replaced with docbook-reference-plugin in
previous commits.)

This commit merely removes the code to prevent confusion 
for other developers in the future.