Remove custom icon selection for trash

Summary: CCBUG:  391200

Test Plan:
1. Open Dolphin
2. Edit trash in places item
Result: No custom icon option available

Reviewers: ngraham, #dolphin, broulik, pino, elvisangelaccio, cfeck

Reviewed By: ngraham, #dolphin, elvisangelaccio, cfeck

Subscribers: rkflx, cfeck, elvisangelaccio, pino, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D14378