powerpc: Remove duplicate definition for ppc_msgsnd_sync()

Remove duplicate definition of ppc_msgsnd_sync() introduced in commit
b87ac021 ("powerpc: Introduce msgsnd/doorbell barrier primitives").

No functional change intended.

Signed-off-by: Gautam Menghani <gautam@linux.ibm.com>
Reviewed-by: Ritesh Harjani (IBM) <riteshh@linux.ibm.com>
[maddy: Updated commit message to fixed checkpatch.pl warning]
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250813122319.62278-1-gautam@linux.ibm.com