mips: drop __pXd_offset() macros that duplicate pXd_index() ones

The __pXd_offset() macros are identical to the pXd_index() macros and there
is no point to keep both of them. All architectures define and use
pXd_index() so let's keep only those to make mips consistent with the rest
of the kernel.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Paul Burton <paulburton@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: Mike Rapoport <rppt@kernel.org>