Remove similar_to functionality

[koetsier]

This is not documented and not used so it is better to remove.

Jira: https://gov-uk.atlassian.net/browse/SCH-2062

[emmalowe]

I think it would be helpful to add some context to the commit message. Here's what I found:

• This feature was originally added to search-api to support a prototype navigation app, which pulled in related links using the "more like this" feature in elasticsearch:
  More like this #747
  More like this govuk-content-navigation#120

• The "similar" related links feature got moved to a temporary navigation components gem called govuk_navigation_helpers, which then got moved into govuk_publishing_components.

• Eventually, the "similar" related links feature got retired because it wasn't performing well, so that we were showing only curated links.
  Remove rummager powered sidebar govuk_publishing_components#530

[emmalowe]

Code changes look good to me. Just the comment about putting the context on the commit message, and then a small nitpick if you feel like addressing that.

[emmalowe]

nit-pick: this file should be called sort_spec.rb really