Remove MimeType related code

Summary:
This is uneeded and we can create and use the mimetype
directly inside of the DetachableTabBar as nothing else
needs to know about it.

Test Plan: Compile and runs. Unused code.

Reviewers: hindenburg, sandsmark, ngraham

Reviewed By: hindenburg

Subscribers: konsole-devel

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D15378