client: remove O_LAZY

The once-upon-a-time unique O_LAZY value I chose forever ago is not
O_NOATIME, which means that some clients were choosing relaxed
consistency without meaning to.

It is highly unlikely that a real O_LAZY will ever exist, and we can
select it in the ceph case with the ioctl or libcephfs call, so drop
any support for doing this via open(2) flags.

Remove the outdated doc/lazy_posix.txt file.

Backport: cuttlefish
Signed-off-by: Sage Weil <sage@inktank.com>