fix!: re-remove block and workspace drag surfaces

[BeksOmega]

Reversion of reversion #6888

Reapplication of #6758

(Re) Fixes #6160

The details

Proposed changes

Removes the block drag surface and the workspace drag surface from Blockly.

Reason for changes

Both of these were implemented as a performance optimization, but have since become a performance sink. Removing these gives us a significant boost in FPS.

Breaking changes / updating / upgrading

Unless you were accessing the drag surface directly, this change should not affect you.

If you were accessing the drag surface directly, you should now access either the workspace svgGroup_ or the svgBlockCanvas_. Here is an example PR that updates the scroll-options plugin: https://github.com/google/blockly-samples/pull/1666/files

Note that we are planning to create more formal get methods to replace these properties in the future (see #7157). If you have opinions about this please feel welcome to leave comments there!

[rockyxia]

Dragging a blocklyDraggable element onto a blocklyToolboxDiv element will be obstructed by blocklyToolboxDiv, making it impossible to elevate the blocklyDraggable element level. Originally, raising the blocklyBlockDragSurface element level was sufficient. How can we solve this problem?

blocklyDraggable元素拖拽到blocklyToolboxDiv元素会被blocklyToolboxDiv遮挡，无法提升blocklyDraggable元素层级，原来通过提升blocklyBlockDragSurface元素层级即可，如何解决这个问题？