[LU-18442] Trivial MDC cleanup

• Type: Improvement
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Lustre 2.17.0
• Severity: 3
• Assignee: Patrick Farrell
• Reporter: Patrick Farrell

Description

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.