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.

(cherry picked from commit 8da21b0ec18cf9306a8c1b3410d5c6ab36acdd21)

https://github.com/krb5/krb5/commit/2b0a29f5232f41ad7127e46469a1f1de9dfaddf2
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: 2b0a29f5232f41ad7127e46469a1f1de9dfaddf2
Branch: krb5-1.11
 doc/admin/conf_files/kdc_conf.rst |    4 ----
 1 files changed, 0 insertions(+), 4 deletions(-)