readme: remove README.PTHREADED_UBIK

readme: remove README.PTHREADED_UBIK

We enabled pthreaded ubik by default in commit 27cb0d38885428474b0d4287,
and it is no longer considered beta or experimental.  There is no longer
a need for separate documentation of it, and adjust the options
listing in INSTALL accordingly.

[kaduk@mit.edu: adjust for the changed default behavior.]

Change-Id: Ib1315e55c1e00bdae0f55f0f8446f5a2c3d9671f
Reviewed-on: http://gerrit.openafs.org/10978
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>