glx: Remove unused bswap_CARD64

GCC warned about it:

../../../glx/indirect_dispatch_swap.c:85:1: warning: ‘bswap_CARD64’ defined but not used [-Wunused-function]
   85 | bswap_CARD64(const void *src)
      | ^~~~~~~~~~~~
4 jobs for glx-drop-bswap_CARD64 in 6 minutes and 11 seconds (queued for 1 second)