[containments/desktop] Delete the "Tweaks" page and its options

Summary:
With the new global edit mode, both of the options on the Tweaks page become unnecessary:

• The option to hide the toolbox becomes dangerous because it breaks global edit mode
• The option to toggle press-and-hold is unnecessary with the far superior global edit mode

Test Plan:

• Right-click desktop > Configure Desktop > No "Tweaks" page at the bottom of the list:
  
• Widget moving, resizing, and editing behaviors in side and out of edit mode are unchanged
• You can no longer turn off the toolbox and break edit mode
• You can no longer toggle the press-and-hold checkbox to show widget handles by hovering over widgets for a few seconds

Reviewers: mart, GB_2, Plasma, hein

Reviewed By: mart, Plasma

Subscribers: fabianr, broulik, plasma-devel

Tags: Plasma

Maniphest Tasks: T10402

Differential Revision: https://phabricator.kde.org/D25055