slirp: Remove unused code for bad sprintf

Neither DECLARE_SPRINTF nor BAD_SPRINTF are needed for QEMU.

QEMU won't support systems with missing or bad declarations
for sprintf. The unused code was detected while looking for
functions with missing format checking. Instead of adding
GCC_FMT_ATTR, the unused code was removed.

Cc: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
