Remove unused line from twig_render_template()

This issue has novice tasks. If you are an experienced core developer and have multiple commit mentions, please review novices' work on these tasks rather than doing them yourself. Feedback from experienced contributors is valued.

Problem/Motivation

The following line from twig.engine is no longer needed now that TwigReference is gone via #2114563: Remove TwigReference with the help of 'without' filter replacing 'show'/'hide' functions.. When dumping all available variables in a Twig template _references shows up.

  $variables['_references'] = array();

Proposed resolution

Remove the line.

Remaining tasks

• Write patch (novice)
• Review patch to check it fixes the issue, the change is properly documented and for coding standards.

User interface changes

n/a

API changes

n/a