Remove 8-bit support from documentation

We removed support in the code ages ago, but overlooked this part of the
documentation. Also remove some dead code in Xvnc on the same theme.

unix/xserver/hw/vnc/xvnc.c

@@ -1577,16 +1577,6 @@ vfbScreenInit(ScreenPtr pScreen, int argc, char **argv)
     miSetPixmapDepths();
 
     switch (pvfb->fb.depth) {
-    case 8:
-	miSetVisualTypesAndMasks (8,
-				  ((1 << StaticGray) |
-				  (1 << GrayScale) |
-				  (1 << StaticColor) |
-				  (1 << PseudoColor) |
-				  (1 << TrueColor) |
-				  (1 << DirectColor)),
-				  8, PseudoColor, 0, 0, 0);
-	break;
     case 16:
 	miSetVisualTypesAndMasks (16,
 				  ((1 << TrueColor) |