removes default value from dont_change_homepage argument

[andres-blanco]

This PR is a follow up to this other one. We need to remove the default value for the dont_change_homepage argument since the checks that are already in place check for the existence of this argument not the value. Changing this logic would end up being more complex down the road.

For more context see: paYKcK-1Mb-p2#comment-1395

Testing instructions:

1. Set up a Jurassic Ninja site.
2. Connect it to WP.com.
3. Connect to your JN site through ssh.
4. Add this snippet to the bottom of /srv/users/[USER]/apps/[USER]/public/wp-load.php.

add_action('jetpack_pre_switch_theme', function($arg1, $args ) {
        error_log( var_export( $arg1, true ) );
        error_log( var_export( $args, true ) );
}, 10, 2);

5. Run tail -F /srv/users/[USER]/apps/[USER]/public/wp-content/debug.log.
6. Go to the calypso.live link on this PR and select your JN site.
7. Go to the theme showcase.
8. Select the twenty twenty theme and press Activate
9. Verify this is the output in the log when selecting the preserve option:

[12-Jul-2022 15:01:49 UTC] 'twentytwenty'
[12-Jul-2022 15:01:49 UTC] array (
  'theme' => 'twentytwenty',
  'dont_change_homepage' => true,
)

10. Verify this is the output in the log when selecting the replace homepage option:

[12-Jul-2022 15:01:49 UTC] 'twentytwenty'
[12-Jul-2022 15:01:49 UTC] array (
  'theme' => 'twentytwenty',
)

Does this pull request change what data or activity we track or use?

No.

Related to Automattic/wp-calypso#56869

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Jetpack plugin:

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[nelsonec87]

Tested and it works!