[JAVA: okhttp-gson, rest-assured, retrofit2] Don't generate Jackson import when serialization library is GSON #750

Job	Run time
Build sbt/Scala client, servers (samples/client/petstore/java/okhttp-gson)	31s
Build sbt/Scala client, servers (samples/client/petstore/scalaz)	2m 27s
Build sbt/Scala client, servers (samples/client/petstore/scala-pekko)	42s
Build sbt/Scala client, servers (samples/server/petstore/scala-lagom-server)	5m 30s
Build sbt/Scala client, servers (samples/server/petstore/scala-play-server)	4m 47s
Build sbt/Scala client, servers (samples/server/petstore/scala-akka-http-server)	3m 21s
Build sbt/Scala client, servers (samples/server/petstore/scala-pekko-http-server)	38s
Build sbt/Scala client, servers (samples/server/petstore/scalatra)	5m 52s
Build sbt/Scala client, servers (samples/server/petstore/scala-finch)	4m 56s
Build sbt/Scala client, servers (samples/server/petstore/scala-http4s-server)	43s
Build sbt/Scala client, servers (samples/server/petstore/scala-cask)	1m 3s
	30m 30s

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