Remove outdated doc about using deprecated-v1 (#3543)

Signed-off-by: Shriram Rajagopalan <shriramr@vmware.com>

Mirrored from https://github.com/envoyproxy/envoy @ f9cc4e2c3916d1994514d22cfa2eb901589d4726

envoy/config/filter/README.md

@@ -2,23 +2,3 @@ Protocol buffer definitions for filters.
 
 Visibility of the definitions should be constrained to none except for
 shared definitions between explicitly enumerated filters (e.g. accesslog and fault definitions).
-
-## NOTE
-
-If a filter configuration is not captured in the proto specification, you
-can still supply plain JSON configuration objects for such filters by
-setting the `"deprecated_v1"` field to true in the filter's
-configuration. For example,
-
-```json
-{
- "name": "envoy.rate_limit",
-  "config": {
-    "deprecated_v1": true,
-     "value": {
-       "domain": "some_domain",
-        "timeout_ms": 500
-       }
-    }
- }
-```