[swiftSyntax] Remove validate methods

The methods were never executed because DEBUG was never defined in
normal builds and the only way to create nodes is through generated
factory methods which provide the same safety `validate` was supposed to
ensure at the interface level.

utils/gyb_syntax_support/Node.py

@@ -58,7 +58,7 @@ class Node(object):
 
     def requires_validation(self):
         """
-        Returns `True` if this node should have a `valitate` method associated.
+        Returns `True` if this node should have a `validate` method associated.
         """
         return self.is_buildable()