Deprecate share email thread (#9319)

## Context

Following this https://github.com/twentyhq/twenty/issues/4199
This has not been fully implemented, after 5months of dead code I'm
removing the feature for the time being until we re-prioritise the
feature (unlikely during these next 6 months) to keep the codebase a bit
cleaner (no need to maintain dead features)

Feel free to reopen / revert this PR once feature is ready

## Test
locally after importing emails

packages/twenty-front/src/modules/activities/emails/right-drawer/hooks/useRightDrawerEmailThread.ts

@@ -14,9 +14,7 @@ import { useFindManyRecords } from '@/object-record/hooks/useFindManyRecords';
 import { useFindOneRecord } from '@/object-record/hooks/useFindOneRecord';
 import { viewableRecordIdState } from '@/object-record/record-right-drawer/states/viewableRecordIdState';
 import { useUpsertRecordsInStore } from '@/object-record/record-store/hooks/useUpsertRecordsInStore';
-import { useIsFeatureEnabled } from '@/workspace/hooks/useIsFeatureEnabled';
 import { isDefined } from 'twenty-ui';
-import { FeatureFlagKey } from '~/generated/graphql';
 
 export const useRightDrawerEmailThread = () => {
   const viewableRecordId = useRecoilValue(viewableRecordIdState);
@@ -38,14 +36,9 @@ export const useRightDrawerEmailThread = () => {
     },
   });
 
-  const isMessageThreadSubscribersEnabled = useIsFeatureEnabled(
-    FeatureFlagKey.IsMessageThreadSubscriberEnabled,
-  );
-
   const FETCH_ALL_MESSAGES_OPERATION_SIGNATURE =
     fetchAllThreadMessagesOperationSignatureFactory({
       messageThreadId: viewableRecordId,
-      isSubscribersEnabled: isMessageThreadSubscribersEnabled,
     });
 
   const {