Skylark: Remove static type checks

--
MOS_MIGRATED_REVID=91175430
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/NotExpression.java b/src/main/java/com/google/devtools/build/lib/syntax/NotExpression.java
index 5a13e79..2fc3fe0 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/NotExpression.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/NotExpression.java
@@ -44,9 +44,7 @@
   }
 
   @Override
-  SkylarkType validate(ValidationEnvironment env) throws EvalException {
-    // Don't need type check here since EvalUtils.toBoolean() converts everything.
+  void validate(ValidationEnvironment env) throws EvalException {
     expression.validate(env);
-    return SkylarkType.BOOL;
   }
 }