Kill DoChangeGC in favor of dixChangeGC.

This doesn't change any behavior, but it isn't clear whether NullClient
is correct in all cases. As ajax says,

> For most of these changes, I think it's correct to use NullClient,
> since they are server-initiated changes and should not fail for (eg)
> xace reasons. ... At any rate, you're certainly not changing any
> semantics by leaving them all as NullClient, so this patch can't be
> more wrong than before.

Signed-off-by: Jamey Sharp <jamey@minilop.net>
Reviewed-by: Keith Packard <keithp@keithp.com>

include/gc.h

@@ -93,12 +93,6 @@ extern _X_EXPORT int ChangeGC(
     BITS32 /*mask*/,
     XID* /*pval*/);
 
-extern _X_EXPORT int DoChangeGC(
-    GCPtr/*pGC*/,
-    BITS32 /*mask*/,
-    XID* /*pval*/,
-    int /*fPointer*/);
-
 typedef union {
     CARD32 val;
     pointer ptr;