cdx: remove unused struct 'cdx_mcdi_copy_buffer'

cdx: remove unused struct 'cdx_mcdi_copy_buffer'

'cdx_mcdi_copy_buffer' has been unused since the original
commit eb96b740192b ("cdx: add MCDI protocol interface for firmware
interaction").

Looking at lkml,  it was used in the V1 posting but was removed
somewhere before V6.

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Acked-by: Nipun Gupta <nipun.gupta@amd.com>
Link: https://lore.kernel.org/r/20240530233436.224461-1-linux@treblig.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>