ci: Remove migrated libraries from downstream protobuf check #644

Job	Run time
filter	1m 6s
build(21) except self-service clients	12m 36s
build(8) for showcase	9m 49s
build(8) for gapic-generator-java	11m 41s
build(8) except for gapic-generator-java	3m 53s
showcase (25)	10m 19s
showcase (17)	11m 7s
showcase (11)	10m 30s
build(25) except self-service clients	12m 16s
showcase (21)	10m 36s
unmanaged_dependency_check	6m 49s
showcase-clirr	4m 58s
dependency_analyzer_unit_test	1m 28s
build (11)	13m 44s
gapic-generator-java-bom	2m 18s
build (17)	13m 41s
	2h 16m 51s

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