Remove in-agent tracking of child process perf stats

[ranweiler]

• Remove the process_stats custom event
• Stop "manually" recording libfuzzer child process resource usage in the onefuzz-agent process
• Remove the onefuzz::system module and our dependency on sysinfo

We can use Azure Monitor perf counters to get the same information, in a more robust, queryable, and less intrusive way. Using sysinfo in our long-lived parent process setting is prone to handle leaks on Windows, which caused task-failing OOM scenarios with certain fuzzer targets and target behaviors.

Due to the removal of the custom event, this is a breaking change.

Fixes #1064.