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.

(cherry picked from commit e972ce069dd45aad74e4f4853570ab27cfea05c2)

https://github.com/krb5/krb5/commit/ce3ceacd52ac912869549e245a498c2c2c42668b
Author: Tom Yu <tlyu@mit.edu>
Commit: ce3ceacd52ac912869549e245a498c2c2c42668b
Branch: krb5-1.10
 README                            |   12 ++++--------
 src/plugins/kdb/db2/libdb2/README |    3 +--
 2 files changed, 5 insertions(+), 10 deletions(-)