EraseButton: add default for accessibleName.

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Related to phetsims/models-of-the-hydrogen-atom#67 and phetsims/sun#901 ...

EraseButton should have a default for accessibleName, rather than every sim adding the same value. In MOTHA phetsims/models-of-the-hydrogen-atom#67, we identifed 'Erase' as an appropriate default. This string should be added to scenery-phet-strings_en.json. We did not identfy a default for helpText, since it is liable to be specific to each sim (what is being erased, etc.)