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

arch/mips/include/asm/fpu.h

@@ -30,9 +30,6 @@
 #include <asm/mips_mt.h>
 #endif
 
-struct sigcontext;
-struct sigcontext32;
-
 extern void _save_fp(struct task_struct *);
 extern void _restore_fp(struct task_struct *);