Remove views_view_summary.html.twig, call use item-list.html.twig instead

Problem/Motivation

We have two identical templates in core, views_view_summary.html.twig and item_list.html.twig. There's no reason for two. More code to maintain, and less consistent markup.

Proposed resolution

Remove views_view_summary.html.twig, replace usage with item_list.html.twig instead

Remaining tasks

update all usage of theme('views_view_summary') with theme('item_list__views_view_summary')
remove the views_view_summary template file
remove the 'views_view_summary preprocess function

User interface changes

None.

API changes

Removal of 'views_view_summary.html.twig + preprocess

Related Issues

#1813426: [meta] Consolidate all item list templates and add theme_hook_suggestions
#1939064: Convert theme_links() to Twig