Remove prom_process_collector.go and report process metrics from process reporter

[robskillington]

prom_process_collector.go was introduced to alleviate the fact we removed the default Prometheus collector due to it's excessive load when counting open FDs with processes that had a large number of FDs.

A modified version of the Prometheus process collector was restored to add back stats removed from the fix to avoid reporting open FDs from the Prometheus collector. For more see #1633.

We should remove this modified version of the Prometheus process collector once the M3 process reporter reports the same information.