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 18a536f1f8 ("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>

docs/devel/tcg-icount.rst

@@ -62,12 +62,6 @@ To deal with this case, when an I/O access is made we:
   - re-compile a single [1]_ instruction block for the current PC
   - exit the cpu loop and execute the re-compiled block
 
-The new block is created with the CF_LAST_IO compile flag which
-ensures the final instruction translation starts with a call to
-gen_io_start() so we don't enter a perpetual loop constantly
-recompiling a single instruction block. For translators using the
-common translator_loop this is done automatically.
-  
 .. [1] sometimes two instructions if dealing with delay slots  
 
 Other I/O operations