kimg_rgb: optimize away QRegExp and QString::fromLocal8Bit.

Summary:
The code is even simpler this way.

Found by using heaptrack.

Test Plan: the unittest for rgb still passes.

Reviewers: cfeck

Reviewed By: cfeck

Subscribers: jtamate, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D15890