[64143110] Remove icinga-web PPA
[64143110] Remove icinga-web PPA

The `icinga-web` package provides a web 2.0 replacement to `icinga-cgi`.
@bobwalker doesn't like this new-fangled stuff, so we're not using it.

The `sources.list.d` entry will be purged by our current APT settings.

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