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

arch/riscv/kernel/traps.c

@@ -25,8 +25,6 @@
 
 int show_unhandled_signals = 1;
 
-extern asmlinkage void handle_exception(void);
-
 static DEFINE_SPINLOCK(die_lock);
 
 void die(struct pt_regs *regs, const char *str)