Sema: Remove unnecessary AtRethrows diagnostic

The attribute is already declared OnProtocol only, so this
diagnostic is unreachable.

Author: slavapestov

include/swift/AST/DiagnosticsSema.def

@@ -2994,8 +2994,6 @@ ERROR(override_rethrows_with_non_rethrows,none,
       "be 'rethrows'", (bool))
 ERROR(rethrows_without_throwing_parameter,none,
       "'rethrows' function must take a throwing function argument", ())
-ERROR(rethrows_attr_on_non_protocol,none,
-      "@rethrows may only be used on 'protocol' declarations", ())
 
 ERROR(autoclosure_function_type,none,
       "@autoclosure attribute only applies to function types",

lib/Sema/TypeCheckAttr.cpp

@@ -2062,14 +2062,7 @@ void AttributeChecker::visitRethrowsAttr(RethrowsAttr *attr) {
   attr->setInvalid();
 }
 
-void AttributeChecker::visitAtRethrowsAttr(AtRethrowsAttr *attr) {
-  if (isa<ProtocolDecl>(D)) {
-    return;
-  }
-
-  diagnose(attr->getLocation(), diag::rethrows_attr_on_non_protocol);
-  attr->setInvalid();
-}
+void AttributeChecker::visitAtRethrowsAttr(AtRethrowsAttr *attr) {}
 
 /// Collect all used generic parameter types from a given type.
 static void collectUsedGenericParameters(