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-server/src/engine/core-modules/feature-flag/enums/feature-flag-key.enum.ts

@@ -7,7 +7,6 @@ export enum FeatureFlagKey {
   IsFreeAccessEnabled = 'IS_FREE_ACCESS_ENABLED',
   IsFunctionSettingsEnabled = 'IS_FUNCTION_SETTINGS_ENABLED',
   IsWorkflowEnabled = 'IS_WORKFLOW_ENABLED',
-  IsMessageThreadSubscriberEnabled = 'IS_MESSAGE_THREAD_SUBSCRIBER_ENABLED',
   IsSSOEnabled = 'IS_SSO_ENABLED',
   IsGmailSendEmailScopeEnabled = 'IS_GMAIL_SEND_EMAIL_SCOPE_ENABLED',
   IsAnalyticsV2Enabled = 'IS_ANALYTICS_V2_ENABLED',