chore: remove extraneous lines from prometheus blackbox exporter overrides (#1283)

the removed lines don't do anything, no value .selfMonitor exists as a top
level override for this to have any effect.

Additionally, I verified that it doesn't just belong under some other key by
looking at the previous overrides:
https://github.com/rackerlabs/genestack/blob/c2714f05275cdff06788d199fa64e650fd3faaae/base-helm-configs/prometheus-blackbox-exporter/values.yaml
^F/find selfMonitor, we didn't have any '.something.selfMonitor = enabled'

These just look like mysterious extraneous lines I got in there somehow since
I previously verified that the pruned overrides produced an identical chart
rendering in #1280

JIRA:OSPC-1615

Author: awfabian-rs

base-helm-configs/prometheus-blackbox-exporter/prometheus-blackbox-exporter-helm-overrides.yaml

@@ -14,6 +14,3 @@ config:
           - 202
           - 204
           - 300
-
-selfMonitor:
-  enabled: true