Remove vestiges of "-diagnose-generic-extensions".

We're not going to require the type parameters to be redeclared on
extensions of generic types, so take away the staging option and
diagnostic.

Swift SVN r26854

lib/Frontend/CompilerInvocation.cpp

@@ -630,9 +630,6 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.DebugConstraintSolver |= Args.hasArg(OPT_debug_constraints);
   Opts.DebugGenericSignatures |= Args.hasArg(OPT_debug_generic_signatures);
 
-  Opts.DiagnoseGenericExtensions
-    |= Args.hasArg(OPT_diagnose_generic_extensions);
-
   Opts.DebuggerSupport |= Args.hasArg(OPT_debugger_support);
   if (Opts.DebuggerSupport)
     Opts.EnableDollarIdentifiers = true;