Terrain Overlays cannot be hidden

Description:

Overlays in 23.1 should always be user-hideable, via the right-click-on-header menu, per documentation: Unity - Manual: Display or hide an overlay (unity3d.com)

Also, per Unity Standards: [Internal link]

Repro Steps:

1. Activate a Terrain tool
2. Attempt to hide any of the overlays opened by the tool

Actual Results:

The overlay cannot be hidden

Expected Results:

The overlay can be hidden