fix(run): remove unused region tags

[glasnt]

Removes unused region tags:

eventarc_generic_dockerfile
eventarc_pubsub_dockerfile
cloudrun_grpc_dockerfile
run_grpc_dockerfile

• Please merge this PR for me once it is approved

[snippet-bot]

Here is the summary of changes.

You are about to delete 6 region tags.

• eventarc/generic/Dockerfile:15, tag eventarc_generic_dockerfile
• eventarc/pubsub/Dockerfile:15, tag eventarc_pubsub_dockerfile
• run/grpc-ping/Dockerfile:15, tag cloudrun_grpc_dockerfile
• run/grpc-ping/Dockerfile:16, tag run_grpc_dockerfile
• run/grpc-server-streaming/Dockerfile:15, tag cloudrun_grpc_dockerfile
• run/grpc-server-streaming/Dockerfile:16, tag run_grpc_dockerfile

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