Remove the gpperfmon since we have the alternative metrics collector.

1, remove all gpperfmon codes, including gpmon, gpsmon, gpmmon,
gpperfmon and alert_log.
2, remove all gpperfmon GUCs, `gp_enable_gpperfmon`, `gp_perfmon_segment_interval`,
`gp_perfmon_print_packet_info`, `gpperfmon_port` and `gpperfmon_log_alert_level`.
3, remove the `perfmon` and `stats sender` processes/bgworkers.
4, remove the `apu` and `libsigar` dependencies.