LU-18442 mdc: remove PageChecked usage in mdc

LU-18442 mdc: remove PageChecked usage in mdc

PageChecked is set in mdc_readpage, and it's not clear why.
This was added in an early merge of a bunch of code, but
was never checked anywhere.

It's unclear what this ever did, but it's not doing
anything now.  Remove.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I59d40cc5cb9b437182db04b468f7dc222c275424
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52504
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Mikhail Pershin <mpershin@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>