code quality: remove unreachable return from _get_block_template_file()

[justlevine]

This PR updates to remove an unreachable return $new_template_item; statement from _get_block_template_file(), as $template_type will bail at the top of the function( https://github.com/justlevine/wordpress-develop/blob/ae2dd43b83c6a985ebd7ecf07ba03d3a19c53ca0/src/wp-includes/block-template-utils.php#L324-L326) if it's not a wp_template or wp_template_part.
For the same reasons, the second if() was also redundant and able to be removed.

While this issue was surfaced via PHPStan in #7619 (trac: https://core.trac.wordpress.org/ticket/61175 ), it can be remediated independently of that ticket.

Trac ticket: https://core.trac.wordpress.org/ticket/63268

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[SergeyBiryukov]

Thanks for the PR! Merged in r60328.