Fix unexpected signature folder creation, fix logs.

Summary:
This fixes the very minor creation of an empty folder when preparing a sequence job.
The empty folder is a side-effect of the addition of the sequence prefix to SequenceJob's signature.
It is created by updateSequencePrefix. This differential removes the prefix from the path before creating the destination folder.

Test Plan:
Create a few sequence jobs in the Capture module, with a prefix or with the default empty prefix.
Create a scheduler job in the Scheduler module using that sequence file.
Run the scheduler job, check the storage where capture files were generated.
Without the differential, an empty folder with the same name as the prefix for capture files is created.
With the differential, this empty folder is not created.

Reviewers: mutlaqja, wreissenberger

Reviewed By: mutlaqja

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D14874