Refactor: remove role assignment from Terraform

[angela-tran]

Part of #296

We found that because the pipeline service principal defaults to having Contributor role, it is not able to create the role assignment for the ETL principal on the Storage Container.

After some discussion, we decided we should remove the role assignment from being managed by Terraform / the pipeline, and it'll be a part of the manual creation steps to set up the role assignment.

This PR deletes the role configuration from Terraform and adds documentation for the manual steps.

Pre-merge / post-merge actions

For MST, we need to

• run terraform state rm azurerm_role_assignment.velocity_etl[0] so that Terraform "forgets" about the role and doesn't try to destroy it (see docs on rm).
• After merging, the prod pipeline should succeed, and there's nothing left to do.

For SBMTD,

• there's no pre-merge action to do.
• After merging, the prod pipeline should succeed since now there is no permissions error to run into. The last thing to do is to manually create the role assignment for the ETL service principal. Doing so requires the Owner role on your Azure account in the SBMTD subscription. (This has been added as a task on SBMTD Agency card server setup #297)

[angela-tran]

There's a little more code cleanup to do

[thekaveman]

And let's also delete the unused variables from the agency-specific azure-vars.yml files.

[angela-tran]

Successfully ran the terraform state rm... command for MST prod workspace:

C:\dev\eligibility-server\terraform>terraform state rm azurerm_role_assignment.velocity_etl[0]
Removed azurerm_role_assignment.velocity_etl[0]
Successfully removed 1 resource instance(s).