man: Delete man pages that will migrate to the Linux man-pages project

As discussed face-to-face and by email, there are a number of
man pages in keyutils that document kernel APIs. The more natural
home for these pages is Linux man-pages. I've already migrated
the pages listed below into Linux man-pages, and subsequently
enhanced various pages. They'll be released with the next release of
man-pages (around the end of Feb 2017).

keyrings.7
persistent-keyring.7
process-keyring.7
session-keyring.7
thread-keyring.7
user-keyring.7
user-session-keyring.7
7 files changed