Do not add return type to constructors declaration when editing definition in signature assistant.

BUG: 365420

Reviewers: kfunk, mwolff, #kdevelop, brauch

Reviewed By: mwolff, #kdevelop, brauch

Subscribers: brauch, apol, #kdevelop, kdevelop-devel

Tags: #kdevelop

Differential Revision: https://phabricator.kde.org/D11291

(cherry picked from commit e334d754)