Don't set a null completer on a non-editable combobox

Summary:
This was a no-op before and prints the following warning since 5.13:

Setting a QCompleter on non-editable QComboBox is not allowed.

See also https://codereview.qt-project.org/#/c/243045/4

Test Plan: Start dolphin with/without this patch.

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D20016