mips: remove unused macros

Remove __NR_Linux_syscalls from uapi/asm/unistd.h as
there is no users to use NR_syscalls macro in mips
kernel.

MAX_SYSCALL_NO can also remove as there is commit
2957c9e6 ("[MIPS] IRIX: Goodbye and thanks for
all the fish"), eight years ago.

Signed-off-by: Firoz Khan <firoz.khan@linaro.org>
[paul.burton@mips.com:
 - Drop the removal of NR_syscalls which is used by
   kernel/trace/trace.h.]
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Cc: y2038@lists.linaro.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-arch@vger.kernel.org
Cc: arnd@arndb.de
Cc: deepa.kernel@gmail.com
Cc: marcin.juszkiewicz@linaro.org