remove compatibility test #667

Job	Run time
Checks	4m 15s
Compile Benchmarks	3m 47s
Compile and test (2.12.18, 8)	5m 58s
Compile and test (2.12.18, 11)	6m 6s
Gradle tests	6m 30s
Maven tests	5m 32s
Docs	5m 14s
Compile and test (2.13.12, 8)	6m 38s
Compile and test (2.13.12, 11)	7m 4s
sbt scripted tests (gen-scala-server, 2.12)	9m 43s
Compile and test (3.3.1, 8)	8m 3s
sbt scripted tests (gen-java, 2.12)	5m 29s
Compile and test (3.3.1, 11)	7m 32s
	1h 21m 51s

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