Remove the slirp submodule (i.e. compile only with an external libslirp)

Since QEMU 7.1 we don't support Ubuntu 18.04 anymore, so the last big
important Linux distro that did not have a pre-packaged libslirp has
been dismissed. All other major distros seem to have a libslirp package
in their distribution already - according to repology.org:

          Fedora 35: 4.6.1
  CentOS 8 (RHEL-8): 4.4.0
          Debian 11: 4.4.0
 OpenSUSE Leap 15.3: 4.3.1
   Ubuntu LTS 20.04: 4.1.0
      FreeBSD Ports: 4.7.0
      NetBSD pkgsrc: 4.7.0
           Homebrew: 4.7.0
        MSYS2 mingw: 4.7.0

The only one that was still missing a libslirp package is OpenBSD - but
the next version (OpenBSD 7.2 which will be shipped in October) is going
to include a libslirp package. Since QEMU 7.2 will be published after
OpenBSD 7.2, we should be fine there, too.

So there is no real urgent need for keeping the slirp submodule in
the QEMU tree anymore. Thus let's drop the slirp submodule now and
rely on the libslirp packages from the distributions instead.

Message-Id: <20220824151122.704946-7-thuth@redhat.com>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>