Drop dead gxid_proto.h header

This header is no longer used.

gdk/x11/gxid_proto.h

-#define GXID_CLAIM_DEVICE       1
-#define GXID_RELEASE_DEVICE     2
-
-#define GXID_RETURN_OK          0
-#define GXID_RETURN_ERROR       -1
-
-typedef struct GxidClaimDevice_ GxidClaimDevice;
-typedef struct GxidReleaseDevice_ GxidReleaseDevice;
-typedef struct GxidMessageAny_ GxidMessageAny;
-typedef union GxidMessage_ GxidMessage;
-
-typedef unsigned long GxidU32;
-typedef long GxidI32;
-
-struct GxidClaimDevice_ {
-  GxidU32 type;
-  GxidU32 length;
-  GxidU32 device;
-  GxidU32 window;
-  GxidU32 exclusive;
-};
-
-struct GxidReleaseDevice_ {
-  GxidU32 type;
-  GxidU32 length;
-  GxidU32 device;
-  GxidU32 window;
-};
-
-struct GxidMessageAny_ {
-  GxidU32 type;
-  GxidU32 length;
-};
-
-union GxidMessage_ {
-  GxidMessageAny any;
-  GxidClaimDevice claim;
-  GxidReleaseDevice release;
-};