[SalesForce] You can’t remove design attributes ‘helpURL’ when the component implements a Lightning Page interface

This is a bit tricky Exercise .In fact the error gives an idea on why this showing up .When you have implemented a lightning Interface of AllPageTypes which means you cannot get rid of design file attributes

Step 0 - you might have to delete Lightning Pages that make reference to the component. Otherwise Salesforce might show an error when you try to remove the interface (or fields).

Step 1 - Lets remove the Interface from your code first ,so try removing the implements attribute

     <aura:component controller="FormController" access="global">
     <aura:attribute name="taxRate" type="String" access="global"/>
     ....
          </aura:component>

Step 2 - Remove the Design attribute now .

Step 3 - Remove the Attribute once you have removed the designed reference

Step 4 - Add the implements and other code back .

I have made a blog about how to resolve this issue without having to add a design attribute. Link: http://kevansfdc.blogspot.com/2017/02/does-lightning-component-know-if-it-is.html

The solution I propose is to use a aura enabled method which return whether you are in a site context using the Site class that come natively with Salesforce.

@AuraEnabled
public static boolean isCommunity(){
    Id siteId = Site.getSiteId(); // take a look at the apex class Site, you may find more useful method concerning site/community
    if (siteId != null) {
        return true;
    }
    return false;
}

And simply calling this method through an action in Lightning.

var action = component.get("c.isCommunity");
action.setCallback(this, function(response) {
var isCommunity = response.getReturnValue(); // do any operation needed here
});
$A.enqueueAction(action);

Hope this is useful as an answer. Cheers!