Fix console warnings

[dwanjuki]

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• Does your code follow the WordPress' coding standards?
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Changes proposed in this Pull Request:

Removed the pmpro_courses_block_scripts function hooked on enqueue_block_editor_assets to stop the blocks being registered twice. The register_block_type function already enqueues editor assets based on the contents of block.json. Resolves warnings:

Block "pmpro-courses/my-courses" is already registered.
Block "pmpro-courses/all-courses" is already registered.

Returning a fragment instead of an array of elements in the block edit functions. Resolves:

Each child in a list should have a unique "key" prop.

Added __nextHasNoMarginBottom and __next40pxDefaultSize props to TextControls to adopt new default field spacing/sizing and prevent deprecation warnings.

36px default size for wp.components.TextControl is deprecated
Bottom margin styles for wp.components.TextControl is deprecated

Rebuilt assets.

How to test the changes in this Pull Request:

The deprecation warnings can be triggered by adding a course block to a page. Applying the PR resolves all warnings.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?