Remove stub pkinit_mapping_file code

As contributed, the PKINIT code contained code to read a mapping
filename, but never used the resulting structure variable.  Get rid of
the structure field and the code to populate it.
Do not document pkinit_mapping_file

This feature was never implemented in the PKINIT code as it was
contributed; there was only stub support for reading the filename.

ticket: 7928 (new)
target_version: 1.12.2
tags: pullup