Remove unused Prometheus cache methods from Environment and its concern

What does this MR do and why

Environment#clear_prometheus_reactive_cache! and Environment#cluster_prometheus_adapter are unreachable:

• clear_prometheus_reactive_cache! has no caller anywhere in the repository, ee/ included.
• cluster_prometheus_adapter is called only from that method, so it dies with it.

Environment does not include the PrometheusAdapter concern, so neither method overrides anything: the includes are StrongMemoize, ReactiveCaching, CacheMarkdownField, FastDestroyAll::Helpers, Presentable, NullifyIfBlank and FromUnion.

cluster_prometheus_adapter was also broken independently of being unused. It called ::Gitlab::Prometheus::Adapter#cluster_prometheus_adapter, and that class only defines prometheus_adapter and find_cluster_prometheus_adapter, so the call would raise NoMethodError if anything reached it.

Removing those two leaves PrometheusAdapter#clear_prometheus_reactive_cache! without a caller, because the body being deleted was its only one. This MR removes that method as well, so the change does not leave newly dead code behind.

The cascade stops there. The two neighbours in the concern, query_klass_for and build_query_args, are still called from PrometheusAdapter#query.

I did not spot the cascade myself: the static-analysis job did. Keela reported new unused code on the first revision of this MR, which is exactly what it is for.

How to verify

• Search the repository for clear_prometheus_reactive_cache. Before this MR the only hits are the concern definition, the Environment definition, and the call from the latter into the former.
• Check the include list at the top of app/models/environment.rb.
• Check Gitlab::Prometheus::Adapter in lib/gitlab/prometheus/adapter.rb: no cluster_prometheus_adapter.
• query_klass_for and build_query_args keep their callers in PrometheusAdapter#query.

Spotted by itaruby(to be released), a Ruby type checker I'm working on.