Remove doc references to LiskDAO::loadRelatives()

Summary:
`loadRelatives()` was removed in rP9aa5a52fbd1b4265633df88b337eb95acee9bd85.
Thus remove it from the docs.

Test Plan: None. Optionally, run `./bin/diviner generate`

Reviewers: O1 Blessed Committers, mainframe98, valerio.bozzolan

Reviewed By: O1 Blessed Committers, mainframe98, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25982

src/docs/contributor/n_plus_one.diviner

@@ -63,9 +63,6 @@ are no longer paying the overhead of issuing hundreds of extra queries. This
 will perform much better (although, as with all performance changes, you should
 verify this claim by measuring it).
 
-See also @{method:LiskDAO::loadRelatives} method which provides an abstraction
-to prevent this problem.
-
 = Detecting the Problem =
 
 Beyond reasoning about it while figuring out how to load the data you need, the

src/infrastructure/storage/lisk/LiskDAO.php

@@ -122,9 +122,6 @@
  * @{method:loadAllWhere} returns a list of objects, while
  * @{method:loadOneWhere} returns a single object (or `null`).
  *
- * There's also a @{method:loadRelatives} method which helps to prevent the 1+N
- * queries problem.
- *
  * = Managing Transactions =
  *
  * Lisk uses a transaction stack, so code does not generally need to be aware