_setOwnersExplicit is not used

[MrMcGoats]

_setOwnersExplicit has internal visibility, but is never called. I understand this could still be useful for implementations extending ERC721A, however, the current contract can be used as-is and this function is effectively just wasting gas in its current state.

I think it would make sense to either remove it entirely or add it as an optional extension. Or even possibly add an external function that does nothing other than call _setOwnersExplicit.

[cygaar]

We are going to be adding this as an optional extension

[NeoDigi]

This can probably be close be #34

[cygaar]

Cool, moved to an extension as part of #34