monitor: Drop broken, unused asynchronous command interface

The asynchronous monitor command interface goes back to commit 940cc30
(Jan 2010).  Added a third case to command execution.  The hope back
then according to the commit message was that all commands get
converted to the asynchronous interface, killing off the other two
cases.  Didn't happen.

The initial asynchronous commands balloon and info balloon were
converted back to synchronous long ago (commit 96637bc and d72f32),
with commit messages calling the asynchronous interface "not fully
working" and "deprecated".  The only other user went away in commit
3b5704b.

New code generally uses synchronous commands and asynchronous events.

What exactly is still "not fully working" with asynchronous commands?
Well, here's a bug that defeats actual asynchronous use pretty
reliably: the reply's ID is wrong (and has always been wrong) unless
you use the command synchronously!  To reproduce, we need an
asynchronous command, so we have to go back before commit 3b5704b.
Run QEMU with spice:

    $ qemu-system-x86_64 -nodefaults -S -spice port=5900,disable-ticketing -qmp stdio
    {"QMP": {"version": {"qemu": {"micro": 94, "minor": 2, "major": 2}, "package": ""}, "capabilities": []}}

Connect a spice client in another terminal:

    $ remote-viewer spice://localhost:5900

Set up a migration destination dummy in a third terminal:

    $ socat TCP-LISTEN:12345 STDIO

Now paste the following into the QMP monitor:

    { "execute": "qmp_capabilities", "id": "i0" }
    { "execute": "client_migrate_info", "id": "i1", "arguments": { "protocol": "spice", "hostname": "localhost", "port": 12345 } }
    { "execute": "query-kvm", "id": "i2" }

Produces two replies immediately, one to qmp_capabilities, and one to
query-kvm:

    {"return": {}, "id": "i0"}
    {"return": {"enabled": false, "present": true}, "id": "i2"}

Both are correct.  Two lines of debug output from libspice-server not
shown.

Now EOF socat's standard input to make it close the connection.  This
makes the asynchronous client_migrate_info complete.  It replies:

    {"return": {}}

Bug: "id": "i1" is missing.  Two lines of debug output from
libspice-server not shown.  Cherry on top: storage for the missing ID
is leaked.

Get rid of this stuff before somebody hurts himself with it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
