doc: Remove AkkaSdkTestkit from TestKit javadoc #300

Job	Run time
Basic checks	50s
Build and publish artifacts and plugins locally	3m 27s
Check service docker image contents	1m 1s
Run sbt test	3m 13s
Maven samples (shopping-cart-quickstart, true)	44s
Run annotation processor tests	1m 20s
Maven samples (choreography-saga-quickstart, true)	53s
Maven samples (doc-snippets, false)	32s
Maven samples (view-store, true)	1m 19s
Maven samples (spring-dependency-injection, true)	43s
Maven samples (event-sourced-customer-registry, true)	1m 8s
Maven samples (event-sourced-counter-brokers, docker compose up -d, true)	1m 31s
Maven samples (event-sourced-customer-registry-subscriber, true, ./start-upstream.sh)	1m 6s
Maven samples (key-value-counter, true)	1m 0s
Maven samples (key-value-shopping-cart, true)	1m 1s
Maven samples (key-value-customer-registry, true)	1m 12s
Maven samples (tracing, false)	34s
Maven samples (reliable-timers, true)	46s
Maven samples (transfer-workflow, true)	59s
Maven samples (transfer-workflow-compensation, true)	1m 9s
	24m 28s

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