Remove full support for colour maps

Gets rid of a loooot of code and complexity.

Colour map clients are still supported through an
automatically generated map, but we lose the ability to
develop a client or server that uses colour maps
internally.
diff --git a/vncviewer/X11PixelBuffer.cxx b/vncviewer/X11PixelBuffer.cxx
index 548591e..f834003 100644
--- a/vncviewer/X11PixelBuffer.cxx
+++ b/vncviewer/X11PixelBuffer.cxx
@@ -94,7 +94,7 @@
 }
 
 PlatformPixelBuffer::PlatformPixelBuffer(int width, int height) :
-  FullFramePixelBuffer(display_pf(), width, height, NULL, NULL),
+  FullFramePixelBuffer(display_pf(), width, height, NULL),
   shminfo(NULL), xim(NULL)
 {
   // Might not be open at this point