LibEDID: Remove head index when retrieving an EDID from DisplayConnector

We simply don't need that field anymore, as it was used when one
FramebufferDevice could contain multiple framebuffers within it, each
for a connected screen head.

Userland/Libraries/LibC/sys/ioctl_numbers.h

@@ -42,8 +42,6 @@ struct GraphicsHeadModeSetting {
 };
 
 struct GraphicsHeadEDID {
-    int head_index;
-
     unsigned char* bytes;
     unsigned bytes_size;
 };