riscv: remove unused handle_exception symbol

Since commit 79b1feba ("RISC-V: Setup exception vector early")
exception vectors are setup early and the handle_exception symbol from
the asm files is no longer referenced in traps.c. Remove it.

Signed-off-by: Rouven Czerwinski <rouven@czerwinskis.de>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>