Remove emulator-reverse-proxy (#817)
Remove emulator-reverse-proxy (#817)

Apparently it was only used for `gcloud alpha emulators start` and has
been removed recently.
google-octo/cloud-builders,google-octo/cloud-builders,GoogleCloudPlatform/cloud-builders,GoogleCloudPlatform/cloud-builders,GoogleCloudPlatform/cloud-builders,google-octo/cloud-builders,GoogleCloudPlatform/cloud-builders,GoogleCloudPlatform/cloud-builders,GoogleCloudPlatform/cloud-builders,google-octo/cloud-builders,google-octo/cloud-builders,GoogleCloudPlatform/cloud-builders,google-octo/cloud-builders,google-octo/cloud-builders