Fix the synchronous-mode wallet open logic

BUG: 254198

The wallet opening logic, for the synchronous mode, had a nested
event loops problem, leading to frozen kwalletd. That was because
kwalletd wasn't using qdbus delayed replies. kdelibs used
asynchronous open methods even for the synchronous mode, coupled
with an internal event loop to simulate synchronous mode.
This commit removes that internal event loop, as the kwalletd now
blocks on synchronous wallet open requests.

This commit has the same effect as f8fea3f0 on kdelibs 4.11
but cherry-picking won't work because of KF5 source splitting. I just
redone it here.