MIPS: Drop forward declarations of sigcontext in asm/fpu.h

asm/fpu.h contains forward declarations of struct sigcontext & struct
sigcontext32 which appear to have been unused since commit 137f6f3e
("MIPS: Cleanup signal code initialization"). Remove the dead code.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/21015/
Cc: linux-mips@linux-mips.org