Fix minimum height of cursor/icon theme comboboxes

Summary:
The manually specified min height of the group box was smaller than
the default one, so just remove it.

Test Plan: Tiny comoboxes before, normalsized comboboxes now.

Reviewers: #plasma, apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9457