Remove stub pkinit_win2k code

As contributed, the PKINIT module contained code to read the
pkinit_win2k variable, but never used it.  Get rid of the structure
field and the code to populate it.
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.

ticket: 7932 (new)
target_version: 1.12.2
tags: pullup