Use common MoveDraggableItemsKeyboardHelpSection

[zepumph]

Today over in phetsims/my-solar-system#93 and phetsims/scenery-phet#801, we created and are using a common help section based on the one from Geometric Optics for dragging:

Because this sim has been published, there is going to be some annoyance about translations for the previously GO-specific strings, but it would be best if we used the common code type in this sim.

For next time we are working on this sim!

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Self-assigning, to consider for the 1.3 release #443.

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scenery-phet's MoveDraggableItemsKeyboardHelpSection is now used in keyboard help. I verified that it's identical to what it replaced.

Closing.

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@zepumph said above:

Because this sim has been published, there is going to be some annoyance about translations for the previously GO-specific strings, but it would be best if we used the common code type in this sim.

In the above commit, I removed unused strings. Translators will need to translate the scenery-phet strings used by MoveDraggableItemsKeyboardHelpSection.