Remove code_suggestions_enabled project setting in favor of duo_features_enabled

In !143278 (merged) the duo_features_enabled project setting has been added to control both code suggestions and duo chat access for a project. We should remove the previously implemented code_suggestions project setting as it was never used / documented and is now unnecessary

Implementation plan

To remove this column we need to follow the guidelines for dropping columns and apply it to project_settings.code_suggestions

1. Ignoring the column (release M)
2. Dropping the column (release M+1)
3. Removing the ignore rule (release M+2)