Do not document pkinit_win2k

This variable was never used in the PKINIT code as it was contributed;
there was only code to read its value.

(cherry picked from commit e161636590429aa78a3f04a8bbe0a36dda48a8ab)

https://github.com/krb5/krb5/commit/d5704eb864fd0ebaaf2e76cc161d930e014425fb
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: d5704eb864fd0ebaaf2e76cc161d930e014425fb
Branch: krb5-1.11
 doc/admin/conf_files/krb5_conf.rst |    5 -----
 1 files changed, 0 insertions(+), 5 deletions(-)