Remove ignore rule

What does this MR do and why?

This removes the ignore_column rule for the project_import_level attribute on the namespace_settings model. The column was removed in 16.9.

This is the third and final MR required for the removal of a column. The previous MRs:

• Remove project_import_level column (16.9): !140322 (merged)
• Ignore project_import_level column (16.8): !141188 (merged)

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Related to #388906 (closed)