Clean up PoC customizations in custom subproject

[JonasKunz]

The custom subproject contains some customizations which we implemented for the PoC and which are very likely not needed or might cause harm (e.g. unnecessary performance overhead).

One example is the ElasticMetricExporter class which is applied automatically and forces all metrics to be exported with delta temporality. This causes e.g. that the JVM metrics dashboards break (because they are built for the vanilla OpenTelemetry agent). Even if we later decide that delta is preferred, we should use the existing otel.exporter.otlp.metrics.temporality.preference configuration option to do that and recommend that option for the vanilla-agent aswell.