docs: delete equivalence topic

(cherry picked from commit e8016a2)

Author: james

docs/language/learn-ql/advanced/advanced-ql.rst

@@ -13,5 +13,4 @@ Topics on advanced uses of QL. These topics assume that you are familiar with QL
 -  :doc:`Semantics of abstract classes <abstract-classes>`
 -  :doc:`Choosing appropriate ways to constrain types <constraining-types>`
 -  :doc:`Determining the most specific types of a variable <determining-specific-types-variables>`
--  :doc:`Understanding the difference between != and not(=) <equivalence>`
 -  :doc:`Monotonic aggregates in QL <monotonic-aggregates>`