Remove collapsible_if and collapsible_else_if lints

[umanwizard]

When I added these lints two years ago, the justification I gave was "semantically wrong level of nesting". However, I have now realized that these lints can actually enforce a semantically wrong level of nesting. For example, the following natural code is rejected by the lint.

            let status = if pod_ready {
                ServiceStatus::Ready
            } else 
                if oomed {
                    ServiceStatus::NotReady(Some(NotReadyReason::OOMKilled))
                } else {
                    ServiceStatus::NotReady(None)
                }
            };

Thus, I am now of the opinion that my initial decision to include these lints was a mistake, and would like to remove them.

Discussion in Slack here.