Remove legacy code in integrations after converting everything to the DSL

Summary

This is a placeholder issue to track any code cleanups we can do after all integrations have been converted to the Field DSL in &7652.

You can also use this issue to mark other issues as blocked by / blocking the DSL refactor.

Improvements

• Simplify Integration#secret_fields: https://gitlab.com/gitlab-org/gitlab/blob/7430f54bcd6ba27a265ec1fd252e2d271f623ecd/app/models/integration.rb#L451-456
• Simplify Integrations::ResetSecretFields#exposing_secret_fields: https://gitlab.com/gitlab-org/gitlab/blob/7430f54bcd6ba27a265ec1fd252e2d271f623ecd/app/models/concerns/integrations/reset_secret_fields.rb#L14-18
• Remove prop_accessor call in Integration.boolean_accessor: https://gitlab.com/gitlab-org/gitlab/-/blob/412778c11c69c051f3d730f5328914f922c7264b/app/models/integration.rb#L206-211

Risks

Involved components

Optional: Intended side effects

Optional: Missing test coverage