API Remove silverstripe/campaign-admin integration support #1018

Job	Run time
CI / Generate matrix	13s
CI / Context	1s
CI / Check governance	0s
CI / 8.3 prf-low mariadb phpunit all	3m 7s
CI / 8.3 mysql80 phpunit all	3m 26s
CI / 8.3 mysql84 phpunit all	3m 18s
CI / 8.3 mysql80 phplinting	1m 48s
CI / 8.3 mysql80 endtoend root job1	9m 11s
CI / 8.3 mysql80 endtoend root job2	6m 43s
CI / 8.3 mysql80 endtoend root job3	6m 58s
CI / 8.3 mysql80 js	4m 1s
CI / Dispatch tag patch release	0s
	38m 46s

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