accel/tcg: Remove CF_LAST_IO

In cpu_exec_step_atomic, we did not set CF_LAST_IO, which lead
to a loop with cpu_io_recompile.

But since 18a536f1 ("Always require can_do_io") we no longer
need a flag to indicate when the last insn should have can_do_io set,
so remove the flag entirely.

Reported-by: Clément Chigot <chigot@adacore.com>
Tested-by: Clément Chigot <chigot@adacore.com>
Reviewed-by: Claudio Fontana <cfontana@suse.de>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1961


Signed-off-by: Richard Henderson <richard.henderson@linaro.org>