MIPS: Remove dead define of ST_OFF

Commit 1a3d5957 ("MIPS: Tidy up FPU context switching") removed the
last usage of the macro ST_OFF. Remove the dead code.

Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: trivial@kernel.org
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/15898/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>