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.

Also don't install the krb5-send-pr.1 manpage, but leave it in the
tree for now.  Continue to install other parts of krb5-send-pr for
now, but plan to remove them from the tree in the future.

ticket: 5566
target_version: 1.12.1
tags: pullup