text: Title: Reduce gensim surface area
  Body: Are there subpackages/submodules that we're not maintaining anymore, and could remove?
  
  - [x] summarization
  - [x] HDP
  - [x] wordrank
  - [x] dependency on pattern
  - [x] various wrappers (incl. sklearn) and others.
  - [x] simserver documentation
  - [x] viz
  
  The goal is to reduce the maintenance burden of the project.
  