Revert "Fix overriding non-open class outside module"

This reverts commit 5c3004b, because I'm not convinced that's a good idea.

We should double-check that **not** having `+supportsSecureCoding` declared on this subclass doesn't make it fail to be copy/pasted (i.e. archived/unarchived)—like similar cases happened when running Aztec tests. Or if we need `+supportsSecureCoding`, we need to find a way to override/redefine it across module boundaries…

WordPressEditor/WordPressEditor/Classes/Plugins/WordPressPlugin/Gutenberg/Gutenblock.swift

@@ -10,4 +10,7 @@ class Gutenblock: ParagraphProperty {
     required public init?(coder aDecoder: NSCoder){
         super.init(coder: aDecoder)
     }
+
+    override public class var supportsSecureCoding: Bool { true }
+
 }