MIPS: Remove unused ST_OFF from r2300_switch.S

MIPS: Remove unused ST_OFF from r2300_switch.S

Commit 1a3d59579b9f ("MIPS: Tidy up FPU context switching") removed
usage of ST_OFF, leaving it behind as dead code. Commit 828d1e4e9865
("MIPS: Remove dead define of ST_OFF") then removed the definition of
ST_OFF from r4k_switch.S as a cleanup. However the unused definition of
ST_OFF has been left behind in r2300_switch.S. Remove it.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16239/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>