debian/ceph-base.docs only referred to a README that doesn't exist, so
remove it. Because dpkg-source doesn't reflect deletions from debian/
cf the orig.tar.gz, also remove the file in dh_auto_clean.
Then do away with the removal of the empty override of dh_installdocs;
the main benefit of which here is that debian/copyright gets installed
in all of the built packages, which otherwise lack a copyright
file.
Signed-off-by: Matthew Vernon <mvernon@wikimedia.org>