remove project_import_level column on namespace_settings table

for #358750 (closed) a DB migration added a project_import_level to the namespace_settings table. We don't need this anymore so the column can be dropped.

Three MRs required:

• Ignore column MR: !141188 (merged) (16.8)
• Remove column MR: !140322 (merged) (16.9)
• Remove ignore code: !144662 (merged) (16.10)