Remove redundant Asset Manager environment variables
Remove redundant Asset Manager environment variables

These variables were made redundant by alphagov/asset-manager#328 in
which proxying asset requests to S3 via Nginx becomes the default for
both Mainstream & Whitehall assets.

We will need to follow this change up with another PR to actually remove
the govuk::app::envvar statements altogether.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet