Revert #3400: Reintroduce experimental S2A integration in client libraries grpc transport #9847

Job	Run time
build (11)	14m 52s
build(21) except self-service clients	13m 6s
build (17)	14m 39s
build(8) except for gapic-generator-java	4m 18s
build(8) for gapic-generator-java	11m 11s
compatibility	1m 33s
lint	22s
showcase (11)	4m 28s
showcase (17)	4m 22s
showcase (21)	4m 20s
dependency_analyzer_unit_test	21s
gapic-generator-java-bom	1m 44s
showcase-clirr	3m 13s
showcase-native	8m 48s
unmanaged_dependency_check	4m 3s
	1h 31m 20s

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