set druid.coordinator.kill.pendingSegments.on to true and remove config option

[pdeva]

Description

1. set druid.coordinator.kill.pendingSegments.on to true by default
2. remove config option entirely

Motivation

There is no reason to set it to false and no downside to setting it to true.

If its going to be true always, it makes sense to remove the config option entirely, since it reduces the overhead of understanding one more config for the user.

[stale]

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[stale]

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[xvrl]

this has been fixed as part of #9385