Make tier ignore_read

[c2thorn]

Fixes hashicorp/terraform-provider-google#5775

The tier field is not consistent enough in the backend to be useful. Marking it as ignore_read, and removing the attempt at polling.

Release Note Template for Downstream PRs (will be copied)

dialogflow: Changed `google_dialogflow_agent` to not read `tier` status

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 4 files changed, 7 insertions(+), 73 deletions(-))
Terraform Beta: Diff ( 4 files changed, 7 insertions(+), 73 deletions(-))

[c2thorn]

Fixes hashicorp/terraform-provider-google#5775