cf-removal: drop prautoblogger_cf_image_via_gateway from fresh-activation defaults

The set_default_options() block was still seeding the dead CF Workers AI gateway
toggle on fresh activations. Removed alongside its settings field + migration
delete-sweep so uninstall → install cycles stop re-adding the option.

Residual from commit 21430ce. Caught in local verification before PR open.

includes/class-activator.php

@@ -69,7 +69,6 @@ class PRAutoBlogger_Activator {
 			'prautoblogger_schedule_time'         => '03:00',
 			'prautoblogger_log_level'             => 'info',
 			'prautoblogger_image_nsfw_retry'      => '1',
-			'prautoblogger_cf_image_via_gateway'  => '1',
 		];
 
 		foreach ( $defaults as $key => $value ) {