Remove superfluous code in QDBusConnectionPrivate::sendWithReply

No need to check for the local loop if that's the first thing
QDBusConnectionPrivate::sendWithReplyAsync will do. The side effect is
that this now allocates memory for the QDBusPendingCallPrivate object,
but loopback messages aren't that common to be worth the special casing.

Task-number: QTBUG-43585
Change-Id: Ic5d393bfd36e48a193fcffff13b73756ab802ba2
Reviewed-by: Albert Astals Cid <aacid@kde.org>
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>