[Widget Explorer] Continue using a trash can icon for "Uninstallable" category

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: {F6778140, size=full}

Reviewers: broulik

Reviewed By: broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D20637