target/riscv: Remove exit_tb and lookup_and_goto_ptr

target/riscv: Remove exit_tb and lookup_and_goto_ptr

GDB single-stepping is now handled generically, which means
we don't need to do anything in the wrappers.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>