Remove 'operator.observability.prometheus: true' from 'featureGatesMap' comment

As per https://github.com/open-telemetry/opentelemetry-helm-charts/blob/0bdda7e6b65507833bdd71555c975f7e4706fd34/charts/opentelemetry-operator/UPGRADING.md, the property is no longer supported here, removed in commit f9707d3

Author: Stogas

charts/opentelemetry-operator/values.yaml

@@ -87,7 +87,6 @@ manager:
   # operator.targetallocator.fallbackstrategy: false
   # operator.collector.targetallocatorcr: false
   # operator.sidecarcontainers.native: false
-  # operator.observability.prometheus: false
   # operator.golang.flags: false
   # operator.collector.default.config: false
   ports: