Remove docker collapsing

This was previously disabled in 682074cc due to issues, in particular, it didn't remember original size when uncollapsing, also, once collapsed, prevented changing the width of entire column.

Test Plan

• Open any document
• Confirm that buttons still work correctly:
  • lock and unlock still works
  • float and unfloat still works
  • close still closes

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.