doc: remove link to "breaking changes" wiki

Breaking changes wiki is not updated, so removing the paragraph

PR-URL: nodejs#19795
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/guides/contributing/pull-requests.md

@@ -165,10 +165,6 @@ use `Refs:`.
 contain an explanation about the reason of the breaking change, which
 situation would trigger the breaking change and what is the exact change.
 
-Breaking changes will be listed in the wiki with the aim to make upgrading
-easier.  Please have a look at [Breaking Changes](https://github.com/nodejs/node/wiki/Breaking-changes-between-v4-LTS-and-v6-LTS)
-for the level of detail that's suitable.
-
 Sample complete commit message:
 
 ```txt