chore: [vertexai] revert the change in the VertexAI.Builder constructor #10891

Job	Run time
gapic-bom	1m 37s
consolidate-config	1m 36s
gitignore_correctness	30s
generate-readme	39s
owlbot-py	40s
owlbot-yaml	34s
root-pom	1m 37s
group_id_check_for_maps_libraries	2m 50s
package_name_check	32s
	10m 35s

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