[Widget Explorer] Continue using a trash can icon for deleting delegates

Summary:
After D20623, some icons change a bit. This patch keeps the delegate using a trash can
icon, and it's more semantically correct as well to use `edit-delete` over `list-remove`

Test Plan: {F6778132, size=full}

Reviewers: broulik, GB_2

Reviewed By: broulik, GB_2

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D20638