LU-13814 clio: transient page related cleanups

LU-13814 clio: transient page related cleanups

Add a few more asserts that transient pages are never
passed to certain functions and remove one unused
declaration.

This series has removed a decent bit of code and
bookkeeping activity from the DIO path, even without
getting to the much bigger wins related to removing
cl_page for DIO.

This gets us about a 17% improvement in DIO
performance - note this is a very different node
from the one which gave us 18 GiB/s two years ago.

ior --posix.odirect -w -r -t 256M -b 64G
Without series:
8.5 GiB/s DIO write
9.6 GiB/s DIO read

With series:
10.1 GiB/s DIO write
11 GiB/s DIO read

Test-Parameters: forjanitoronly
Test-Parameters: fortestonly
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Ic90bd349a1ded8c1484dc8be29418adb8f229026