Editor: Simplify block context to eliminate createInnerBlockList

[aduth]

The createInnerBlockList function was originally added as part of #5228 in a compromise to allow a block (at the time, in blocks) to render a BlockList component via InnerBlocks, avoiding a circular dependency† between blocks and editor by passing the higher-order component through via React context.

Between core blocks being moved to their own directory (#6351) and blocks components being merged into editor (#6521), the dependency hierarchy has changed such that InnerBlocks (now in ./editor) can render a BlockList on its own. It only needs the props which are expected by the component (rootUID, renderBlockMenu). These values are simpler to pass through as context, and avoids the need for createInnerBlockList (and related caching) altogether.

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† Where the dependencies would look as:

BlockList (./editor) -> Block (./blocks) -> InnerBlocks (./blocks) -> BlockList (./editor)