ACPI / CPPC: remove redundant mbox_send_message() declaration

Remove a redundant function declaration in cppc_acpi.h for
mbox_send_message().  That function is defined in mailbox_client.h,
which is already included.

Signed-off-by: Timur Tabi <timur@codeaurora.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

include/acpi/cppc_acpi.h

@@ -133,6 +133,5 @@ extern int acpi_get_psd_map(struct cpudata **);
 /* Methods to interact with the PCC mailbox controller. */
 extern struct mbox_chan *
 	pcc_mbox_request_channel(struct mbox_client *, unsigned int);
-extern int mbox_send_message(struct mbox_chan *chan, void *mssg);
 
 #endif /* _CPPC_ACPI_H*/