lib: utils: Remove redundant clint_ipi_sync() declaration

The clint_ipi_sync() was removed long time back hence we remove
redundant declaration from clint.h

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>