Remove ignore_column rule for file_checksum in xray_report model

As part of #479185 (closed), we are removing the xray_reports.file_checksum database column.

This MR removes the ignore_column rule for file_checksum from the xray_report model, following a multi-release approach for safe column removal.

It's the third step in a three-phase process:

1. Ignoring the column (release M) - addressed with !169920 (merged) in %17.6
2. Dropping the column (release M+1) - [%17.7] (issue)
3. Removing the ignore rule (release M+2) - [%17.8]