MDL-59383 core_calendar: css fixes to summary modal body

calendar/templates/event_summary_body.mustache

@@ -26,7 +26,7 @@
         "eventtype": "open",
     }
 }}
-<div class="container">
+<div>
     <h4>{{#str}} when, core_calendar {{/str}}</h4>
     {{#userdate}} {{timestart}}, {{#str}} strftimerecentfull {{/str}} {{/userdate}}
     <br>

calendar/templates/event_summary_modal.mustache

@@ -26,7 +26,7 @@
 }}
 {{< core/modal }}
 {{$footer}}
-<button type="button" class="btn btn-secondary pull-xs-left" data-action="delete">{{#str}} delete {{/str}}</button>
+<button type="button" class="btn btn-secondary" data-action="delete">{{#str}} delete {{/str}}</button>
 <button type="button" class="btn btn-primary" data-action="edit">{{#str}} edit {{/str}}</button>
 {{/footer}}
 {{/ core/modal }}