Remove KRB5_DNS_LOOKUP_KDC

Remove KRB5_DNS_LOOKUP_KDC

It has been unconditionally activated by all supported build systems
for almost two years, and no complaints or issues have been reported.
In particular, aclocal.m4 has had an unconditional AC_DEFINE() since
3d708e55 in 2003, and win-pre.in has unconditionally set KRB5_USE_DNS_KDC
since 17ffebf7 in 2011.

While here, simplify some other DNS conditionals in win-pre.in where
only one branch was ever taken.

ticket: 7691 (new)