slirp: Remove unused variable and unused code

9634d9031c140b24c7ca0d8872632207f6ce7275 disabled unused code.
This patch removes what was left.

If do_pty is 2, the function returns immediately, so any later checks
for do_pty == 2 will always fail and can be removed together with
the code which is never executed. Then variable master is unused and
can be removed, too.

This issue was detected by coverity.

Cc: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
