Get rid of the register mechanism for encoders and decoders

We have no use for it, so let's keep things simple.
diff --git a/common/rfb/HextileEncoder.h b/common/rfb/HextileEncoder.h
index 6b89643..0c475a8 100644
--- a/common/rfb/HextileEncoder.h
+++ b/common/rfb/HextileEncoder.h
@@ -24,11 +24,10 @@
 
   class HextileEncoder : public Encoder {
   public:
-    static Encoder* create(SMsgWriter* writer);
-    virtual bool writeRect(const Rect& r, TransImageGetter* ig, Rect* actual);
-    virtual ~HextileEncoder();
-  private:
     HextileEncoder(SMsgWriter* writer);
+    virtual ~HextileEncoder();
+    virtual bool writeRect(const Rect& r, TransImageGetter* ig, Rect* actual);
+  private:
     SMsgWriter* writer;
   };
 }