riscv: Remove unused asm/signal.h file

When riscv moved to common entry the definition and usage of
do_work_pending was removed.  This unused header file remains.

Remove the header file as it is not used.

I have tested compiling the kernel with this patch applied and saw no
issues.  Noticed when auditing how different ports handle signals
related to saving FPU state.

Fixes: f0bddf5 ("riscv: entry: Convert to generic entry")
Signed-off-by: Stafford Horne <[email protected]>
Reviewed-by: Guo Ren <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Palmer Dabbelt <[email protected]>

Author: stffrdhrn