chore: remove ending comma in `owlbot.py` #5814

Job	Run time
build (11)	12m 37s
build (17)	12m 59s
build(21) except self-service clients	11m 45s
build(8) except for gapic-generator-java	4m 3s
build(8) for gapic-generator-java	9m 25s
compatibility	1m 58s
lint	24s
showcase (11)	4m 13s
gapic-generator-java-bom	1m 51s
showcase-clirr	3m 17s
showcase-native	8m 18s
showcase (17)	3m 58s
unmanaged_dependency_check	4m 15s
	1h 19m 3s

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