slirp: Drop redundant checks from slirp_output

Slirp doesn't invoke slirp[_can]_output before it is initialized. The
motivation for these checks (3b7f5d479c) no longer applies. So drop
them.

Note: slirp_vc will become invalid if the slirp stack is removed during
runtime. But this is no new bug and will be fixed later.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
