Delete unreferenced conn_manager_impl_common.h (#15265)
Delete unreferenced conn_manager_impl_common.h (#15265)

Commit Message: delete unreferenced conn_manager_impl_common.h
Additional Description:
This file doesn't appear in any BUILD files and isn't referenced by any #includes in Envoy.
Risk Level: low
Testing: git grep
Docs Changes: n/a
Release Notes: n/a
Platform Specific Features: n/a

Signed-off-by: Alex Konradi <3ee61dd7715e31b012f8edaf69177fdbdc394d59@google.com>
envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy