API Remove silverstripe/campaign-admin integration support #1731

Job	Run time
CI / Generate matrix	14s
CI / Context	1s
CI / Check governance	0s
CI / 8.3 prf-low mariadb phpunit all	2m 5s
CI / 8.3 mysql80 phpunit all	2m 10s
CI / 8.3 mysql84 phpunit all	2m 19s
CI / 8.3 mysql80 phplinting	1m 41s
CI / 8.3 mysql80 endtoend root job1	10m 26s
CI / 8.3 mysql80 endtoend root job2	8m 6s
CI / 8.3 mysql80 endtoend root job3	8m 45s
CI / 8.3 mysql80 js	5m 50s
CI / Dispatch tag patch release	0s
	41m 37s

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