Remove metrics metric after a canary is deleted

[AlbertoSH]

TL;DR;
Fixes #1029
plus removes some more unneeded metrics

When deleting a canary, its status metric is not updated. Having two canaries deployed, both initialized, metrics endpoint exposes

# HELP flagger_canary_status Last canary analysis result
# TYPE flagger_canary_status gauge
flagger_canary_status{name="canary1",namespace="foo"} 1
flagger_canary_status{name="canary2",namespace="foo"} 1
# HELP flagger_canary_total Total number of canary object
# TYPE flagger_canary_total gauge
flagger_canary_total{namespace="foo"} 2

After deleting one, the metrics are updated to

# HELP flagger_canary_status Last canary analysis result
# TYPE flagger_canary_status gauge
flagger_canary_status{name="canary1",namespace="foo"} 1   # <--- this one was deleted, but not reflected here
flagger_canary_status{name="canary2",namespace="foo"} 1
# HELP flagger_canary_total Total number of canary object
# TYPE flagger_canary_total gauge
flagger_canary_total{namespace="foo"} 1

Deleting the metric feels more appropriate than marking it as deleted, as otherwise the amount of timeseries can grow a lot and, if it's deleted, it means that it's probably not needed anymore