[KComboBox] Properly disable Qt's builtin completer [regression fix]

Summary:
It is set by setLineEdit when the deprecated bool autoCompletion hasn't
previously been set to false. So the commit that removed
setAutoCompletion(false) led to an unwanted QCompleter, indirectly.

Test Plan:
kfind's "Look in" combobox no longer offers completion
from the combo items (preselected, in the lineedit)

Reviewers: broulik

Reviewed By: broulik

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D23995