Remove mentions of krb5-send-pr

Start the process of deprecating krb5-send-pr.  In practice, it causes
frustration for some users, and free-form email is good enough for
most bug reports.

Continue to install krb5-send-pr for now, but plan to remove it from
the tree in the future, probably replaced by a script that instructs
the user to send email manually.

ticket: 5566
target_version: 1.12.1
tags: pullup