Remove redundant conditions in content blocks controller

In the case of the `edit` action, we're using
`load_and_authorize_resource`, which will always return a
`SiteCustomization::ContentBlock`. In the case of
`edit_heading_content_block`, we're using `Budget::ContentBlock.find`,
which always returns a `Budget::ContentBlock` (or raises an exception).

So, in both cases, the condition to assign `@selected_content_block` can
be removed.

app/controllers/admin/site_customization/content_blocks_controller.rb

@@ -34,11 +34,7 @@ class Admin::SiteCustomization::ContentBlocksController < Admin::SiteCustomizati
   end
 
   def edit
-    if @content_block.is_a? SiteCustomization::ContentBlock
     @selected_content_block = @content_block.name
-    else
-      @selected_content_block = "hcb_#{@content_block.heading_id}"
-    end
   end
 
   def update
@@ -75,11 +71,7 @@ class Admin::SiteCustomization::ContentBlocksController < Admin::SiteCustomizati
 
   def edit_heading_content_block
     @content_block = Budget::ContentBlock.find(params[:id])
-    if @content_block.is_a? Budget::ContentBlock
     @selected_content_block = "hcb_#{@content_block.heading_id}"
-    else
-      @selected_content_block = @content_block.name
-    end
     @is_heading_content_block = true
     render :edit
   end