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is used by precisely one thing (producing a warning in a scenario that is obsolete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/AST/Pattern.cpp

@@ -310,7 +310,6 @@ static Pattern *buildImplicitLetParameter(ASTContext &ctx, Identifier name,
   P->setType(tyLoc.getType());
   P = new (ctx) TypedPattern(P, tyLoc, /*Implicit=*/true);
   P->setType(tyLoc.getType());
-  paramDecl->setParamParentPattern(P);
   return P;
 }