LU-11825 clio: Remove pio feature

LU-11825 clio: Remove pio feature

The pio feature was an interesting concept, but never
achieved real use in production, primarily because it was
actually slower than normal i/o for some common workloads,
so it could not be on by default.

It has had a trivial deadlock in it (with truncate) since
release, and this has never been reported by a customer.

The attemp to rewrite readahead to use ptasks was never
fully successful, and is being superseded by a different
implementation.

Given all this, there is no reason to keep the extra
complexity in the already complicated clio code.

Signed-off-by: Patrick Farrell <paf@cray.com>
Change-Id: Ie558db946af94189e35e985e8894706c14ed5f39
Reviewed-on: https://review.whamcloud.com/33912
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>