[v1.6.0 deprecation] Remove features marked deprecated in #2346

[htuch]

#2346 (Listener: Add listener filters.) introduced a deprecation notice for v1.6.0. This issue will track source code cleanup.

[htuch]

@jrajahalme this one is for you. I sent you an invite to the envoyproxy org, if you can join that I can assign in the GH UI. Thanks.

[jrajahalme]

This depends on #1280 being implemented first.

[stale]

This issue has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in the next 7 days unless it is tagged "help wanted" or other activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically closed because it has not had activity in the last 37 days. If this issue is still valid, please ping a maintainer and ask them to label it as "help wanted". Thank you for your contributions.