lib: sbi: Remove sbi_trap_exit() and related code

Over the years, no uses of sbi_trap_exit() have been found so remove
it and also remove related code from fw_base.S and sbi_scratch.h.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Samuel Holland <samuel.holland@sifive.com>
Tested-by: Samuel Holland <samuel.holland@sifive.com>