API Remove silverstripe/campaign-admin integration support #1026

Job	Run time
CI / Generate matrix	19s
CI / Context	1s
CI / Check governance	0s
CI / 8.3 prf-low mariadb phpunit all	3m 7s
CI / 8.3 mysql80 phpunit all	3m 8s
CI / 8.3 mysql84 phpunit all	2m 56s
CI / 8.3 mysql80 phplinting	1m 49s
CI / 8.3 mysql80 endtoend root job1	9m 3s
CI / 8.3 mysql80 endtoend root job2	8m 52s
CI / 8.3 mysql80 endtoend root job3	6m 54s
CI / 8.3 mysql80 js	5m 10s
CI / Dispatch tag patch release	0s
	41m 19s

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