[cs] remove cs.StringBuilder extern and use auto-generated one from .net dll

std/cs/StringBuilder.hx

@@ -1,37 +0,0 @@
-/*
- * Copyright (C)2005-2012 Haxe Foundation
- *
- * Permission is hereby granted, free of charge, to any person obtaining a
- * copy of this software and associated documentation files (the "Software"),
- * to deal in the Software without restriction, including without limitation
- * the rights to use, copy, modify, merge, publish, distribute, sublicense,
- * and/or sell copies of the Software, and to permit persons to whom the
- * Software is furnished to do so, subject to the following conditions:
- *
- * The above copyright notice and this permission notice shall be included in
- * all copies or substantial portions of the Software.
- *
- * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
- * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
- * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
- * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
- * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
- * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
- * DEALINGS IN THE SOFTWARE.
- */
-package cs;
-
-//FIXME this class is here due to seemingly a bug in type resolution inside _std packages.
-//Once this bug is fixed, it will be moved back to the cs.system.text package
-@:native('System.Text.StringBuilder') extern class StringBuilder
-{
-	var Length(default,never):Int;
-
-	function new():Void;
-
-	@:overload(function(char:cs.StdTypes.Char16):cs.StringBuilder {})
-	@:overload(function(str:String, startIndex:Int, len:Int):cs.StringBuilder {})
-	function Append(obj:Dynamic):StringBuilder;
-
-	function ToString():String;
-}

std/cs/_std/StringBuf.hx

@@ -19,15 +19,17 @@
  * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
  * DEALINGS IN THE SOFTWARE.
  */
+import cs.system.text.StringBuilder;
+
 @:coreApi
 class StringBuf {
 
-	private var b : cs.StringBuilder;
+	private var b : StringBuilder;
 
 	public var length(get,never) : Int;
 
 	public inline function new() : Void {
-		b = new cs.StringBuilder();
+		b = new StringBuilder();
 	}
 
 	inline function get_length() : Int {