Remove the ability to map back from a ParamDecl to its enclosing Pattern. This

is used by precisely one thing (producing a warning in a scenario that is obsolete
because we deprecated the entire thing), so the complexity isn't worth it anymore.

lib/Parse/ParsePattern.cpp

@@ -392,9 +392,6 @@ mapParsedParameters(Parser &parser,
       bool isLet = specifierKind == Parser::ParsedParameter::Let;
       param = new (ctx) VarPattern(letVarInOutLoc, isLet, param);
     }
-
-    if (var)
-      var->setParamParentPattern(param);
     return param;
   };