Remove @todo from phpcs.xml.dist linking to closed issue

Problem/Motivation

#1848264: Compare and merge PhpWiki diff*.php with MediaWiki's DairikiDiff.php and DiffEngine.php has been closed and there is an @todo for it that can be removed. However there does remain remnants of DiffEngine which still do not pass coding standards.

Steps to reproduce

Proposed resolution

1/ Remove this from phpcs.xml.dist:

<!-- @todo This rule may be removed when https://www.drupal.org/node/1848264 is resolved. -->

Remaining tasks

• Merge request

User interface changes

API changes

DiffFormatter.php has three public properties which change.

show_header -> $showHeader
leading_context_lines -> $leadingContextLines
trailing_context_lines -> $trailingContextLines
line_stats -> $lineStats

Data model changes

Release notes snippet