Remove prefect-agent as a possible work pool type

[desertaxle]

Removes prefect-agent as a selectable work pool type. PrefectAgent has been removed for the 3.0 release, so this is no longer needed. The collection registry will still contain data for the prefect-agent work pool type until PrefectAgent is removed from the 2.x line.

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[serinamarie]

LGTM!