Avoid second password prompt appearing

To authorise kio-admin a password prompt appears. Pressing cancel in it should also cancel any similar requests issued at a similar time from a similar source. This mostly worked as expected, however sometimes based on a race condition two password prompts could slip through.

This commit attempts to make this work reliably. In my testing in Dolphin I was no longer able to encounter a second password prompt after cancelling the first one.

This commit removes a time frame from the code in which a variable is temporarily and unnecessarily empty.