Removed Separators in Layers Docker

Removed the separator lines in the layers docker, now looking quite a lot cleaner!

Test Plan

Create a new document, then create multiple layers. The changes should be relatively clear! Here's how it looks:

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.