Remove unwanted OptaPlanner references from website sources

There are still some unwanted references to OptaPlanner in the Drools website sources. For example the https://github.com/droolsjbpm/drools-website/blob/master/_config/site.yml contains keywords related to OptaPlanner rather than Drools.

There are probably others. We need to fix those.

Note: not every reference is necessarily unwanted. There may be some links or other info about OptaPlanner that we want to keep there.