[openapi] Avoid unused imports for generated security schemas

[DMokhnatkin]

Now it is impossible to check if each import is required inside generated security schemas, because muschtache doesn't provide this functionality. That's why lines with " var _ = ... " are generated. It allows to avoid unused import errors. This should be fixed: if openapi schema doesn't contain definition for some security schema, it should't be generated and import should not be added to this file.