QTcpServer already has a list of pending connections and emits newConnection after calling incomingConnection

Summary:
No need to emit newConnection in Server::incomingConnection because QTcpServer will emit it after incomingConnection returns
QTcpServer also already has a list of pending connections so use that

Test Plan: Send a file from desktop to android and observe that newConnection is only called once

Reviewers: KDE Connect, albertvaka

Reviewed By: KDE Connect, albertvaka

Subscribers: kdeconnect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D17009