Remove some dead code and remove an unneeded comment.

Author: gottesmm

include/swift/AST/DiagnosticsSema.def

@@ -8231,14 +8231,11 @@ ERROR(attr_abi_incompatible_with_silgen_name,none,
 //===----------------------------------------------------------------------===//
 // MARK: @execution Attribute
 //===----------------------------------------------------------------------===//
+
 ERROR(attr_execution_concurrent_only_on_async,none,
       "cannot use '@execution(concurrent)' on non-async %kind0",
       (ValueDecl *))
 
-ERROR(attr_execution_concurrent_incompatible_with_global_actor,none,
-      "cannot use '@execution(concurrent)' on %kind0 isolated to global actor %1",
-      (ValueDecl *, Type))
-
 ERROR(attr_execution_concurrent_incompatible_isolated_parameter,none,
       "cannot use '@execution(concurrent)' on %kind0 because it has "
       "an isolated parameter: %1",

lib/Sema/TypeCheckAttr.cpp

@@ -8138,8 +8138,6 @@ class ClosureAttributeChecker
         }
       }
 
-      // Check if we have nonisolated or
-
       return; // it's OK
     }