ci: remove nexus-staging-maven-plugin #18240

Job	Run time
package_name_check	1m 19s
owlbot-yaml	1m 6s
gapic-bom	1m 39s
group_id_check_for_maps_libraries	8m 30s
root-pom	2m 5s
gitignore_correctness	1m 18s
generate-readme	1m 14s
owlbot-py	1m 27s
	18m 38s

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