serial: pmac_zilog: remove unused header

After commit a5ddc498 (serial: pmac_zilog: remove unfinished DBDMA
support), the header is unused and can be removed. So do so.

Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Suggested-by: "Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220602083120.22519-1-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>