MIPS: Remove struct mm_context_t fp_mode_switching field

The fp_mode_switching field in struct mm_context_t was left unused by
commit 8c8d953c

 ("MIPS: Schedule on CPUs we need to lose FPU for a
mode switch") in v4.19, with nothing modifying its value & nothing
waiting on it having any particular value after that commit. Remove the
unused field & the one remaining reference to it.

Signed-off-by: Paul Burton <paul.burton@mips.com>