Remove .conf extension from "INI Files" hl, to determine the highlighter using MIME type

Summary:
BUG: 400290

The extension .conf is very generic, since depending on the context, it can be used with a different language, and assigning it to the "INI Files" highlighter isn't very convenient.
For example, in "Fontconfig" the XML files have a .conf extension.

Therefore, I propose that the .conf extension isn't assigned to any syntax highlighter, so use the MIME type to determine the highlighting mode in those files.

For example, when applying this patch, the Fontconfig files (located in /etc/fonts/) are highlighted as XML by default.

Reviewers: Framework: Syntax Highlighting, dhaumann, cullmann

Reviewed By: Framework: Syntax Highlighting, cullmann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D21288