configure: remove obsolete Solaris ar check

Meson already has its own logic to find the "ar" binary, so remove the
Solaris specific check.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20211007130829.632254-14-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>