Remove unused reference to sphinx-maven-plugin
Remove unused reference to sphinx-maven-plugin

Presto-docs does not use this anymore, this unused reference made me search
and fix the unused code. Removing it to prevent repeat of my future issues.

prestodb/presto,prestodb/presto,zzhao0/presto,zzhao0/presto,prestodb/presto,prestodb/presto,zzhao0/presto,prestodb/presto,zzhao0/presto,zzhao0/presto,prestodb/presto