Revert most of tab bar styling

The styles on the tab bars don't work that well when there are a lot of tabs in a row. The extra styling made the contrast slightly better, but it came at a cost of messing up other things like the text elide and tab padding.

Test Plan

Build and look at it.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• [] I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.