config: moving grpc subscriptions to extensions (#26742)

Risk Level: low
Testing: n/a
Docs Changes: inline
Release Notes: n/a
Signed-off-by: Alyssa Wilk <alyssar@chromium.org>

Mirrored from https://github.com/envoyproxy/envoy @ 522f0551adf70a9fa27edf7088de0c6d2d14d63f

envoy/config/core/v3/config_source.proto

@@ -281,15 +281,3 @@ message ExtensionConfigSource {
   // if they do not match any type URL in the set.
   repeated string type_urls = 4 [(validate.rules).repeated = {min_items: 1}];
 }
-
-// A placeholder proto for the registration for the REST subscription implementation.
-message RestSubscription {
-}
-
-// A placeholder proto for the registration for the filesystem subscription implementation.
-message FilesystemSubscription {
-}
-
-// A placeholder proto for the registration for the filesystem collection subscription implementation.
-message FilesystemCollectionSubscription {
-}