Navigation Link: Remove unit test code for < WP 6.5

[t-hamano]

What?

#58389 now uses get_block_type_variations to build variations for the Navigation Link block. This hook was first introduced in core version 6.5. Therefore, there was a backwards compatibility code to pass unit tests on core versions lower than 6.5.
However, this backwards compatibility code is no longer needed because Gutenberg currently supports core versions 6.6 and above.

Testing Instructions

The unit tests should pass.

[github-actions]

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