Remove old code that removed nested meta queries

[oscarssanchez]

Description of the Change

It seems https://github.com/10up/ElasticPress/pull/2567/files#diff-f5aaa70470d85e353af0ee1d50c73f04d65e2c2a78336a7ac660ead2d8e61d27L542 is no longer needed and as a result is removing nested meta queries.

Closes #1812

Alternate Designs

Possible Drawbacks

Verification Process

1.- Have a meta query like the following:

$meta_query = array(
    array(
        array(
            'key' => 'key1',
            'value' => 'on',
        ),
        array(
            'key' => 'key2'
            'value' => 'off'
        ),
        'relation' => 'AND'
    ),
    array(
        'key' => 'key3',
        'value' => 'red',
    ),
    'relation' => 'OR',
);

2.- Verify debug bar
3.- Meta queryfor key2/off should not be removed

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Changelog Entry

Credits

Props @