readline: don't free completions in readline_free()

Since commit e5dc1a6c, QEMU aborts on exit if completion was used
in the monitor:

*** Error in `obj/ppc64-softmmu/qemu-system-ppc64': double free or
 corruption (fasttop): 0x00000100331069d0 ***

 /home/greg/Work/qemu/qemu-spapr/util/readline.c:514
 /home/greg/Work/qemu/qemu-spapr/monitor.c:586
 /home/greg/Work/qemu/qemu-spapr/monitor.c:4125
 argv=<optimized out>, envp=<optimized out>) at
 /home/greg/Work/qemu/qemu-spapr/vl.c:4795

Completion strings are not persistent accross completions (why would
they?). They are allocated under readline_completion(), which already
takes care of freeing them before returning.

Maybe all completion related bits should be moved out of ReadLineState
to a dedicated structure ?

In the meantime, let's drop the offending lines from readline_free()
to fix the crash.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <151627206353.4505.4602428849861610759.stgit@bahia.lan>
Fixes: e5dc1a6c
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>