Remove unused new diagnostic

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -4121,9 +4121,6 @@ ERROR(asynchandler_async,none,
 ERROR(asynchandler_inout_parameter,none,
       "'inout' parameter is not allowed in '@asyncHandler' function",
       ())
-ERROR(asynchandler_unsafe_pointer_parameter,none,
-      "'%0' parameter is not allowed in `@asyncHandler` function",
-      ())
 ERROR(asynchandler_mutating,none,
       "'@asyncHandler' function cannot be 'mutating'",
       ())