Fix issue Automattic#8546 - instead of removing window.require and wi…

…ndow.define from global scope and thus breaking mailchimp script, set window.define.amd to fool just real amd modules.

Mailchimp script seem to expect usable definitions of require and define, once we remove them it breaks. Since the purpose of this commit is to fix scripts like jquery-ui that will resort to define() if define.amd is defined - it is enough to unset define.amd. This is a very ugly solution, but it works.

modules/shortcodes/mailchimp.php

@@ -199,6 +199,6 @@ static function shortcode( $lcase_attrs ) {
 
 		$displayed_once = true;
 
-		return "\n\n" . '<script type="text/javascript" data-dojo-config="' . esc_attr( implode( ', ', $config_vars ) ) . '">jQuery.getScript( "//downloads.mailchimp.com/js/signup-forms/popup/embed.js", function( data, textStatus, jqxhr ) { require(["mojo/signup-forms/Loader"], function(L) { L.start(' . wp_json_encode( $js_vars ) . ') }); window.require = window.define = undefined; } );</script>' . "\n\n";
+		return "\n\n" . '<script type="text/javascript" data-dojo-config="' . esc_attr( implode( ', ', $config_vars ) ) . '">jQuery.getScript( "//downloads.mailchimp.com/js/signup-forms/popup/embed.js", function( data, textStatus, jqxhr ) { require(["mojo/signup-forms/Loader"], function(L) { L.start(' . wp_json_encode( $js_vars ) . ') }); window.define.amd = undefined; } );</script>' . "\n\n";
 	}
 }