Remove yet another useless assignment

[trasz]

Summary

Remove yet another useless assignment, adding a KASSERT just in case.

[trasz]

Regen.

[arichardson]

sys/riscv/riscv/trap.c

317

Since frame is already stored to td->td_frame, we could drop this argument? And also the mv a0, sp in cpu_exception_handler_user ?
And then just add a struct trapframe *frame = td->td_frame and drop the assert?

[trasz]

sys/riscv/riscv/trap.c

317

That's the plan. I'm just doing this one step at a time, to make it easier to bisect, should it become necessary.