Keep together all the conditions that lead to disabling plugins, and
remove now-dead code.

Since the option was not in SKIP_OPTIONS, it was present twice in
the help message, both from configure and from meson-buildoptions.sh.
Remove the duplication and take the occasion to document the option as
autodetected, which it is.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>