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/generated/graphql.tsx

@@ -117,7 +117,6 @@ export type Billing = {
 
 /** The different billing plans available */
 export enum BillingPlanKey {
-  Base = 'BASE',
   Enterprise = 'ENTERPRISE',
   Pro = 'PRO'
 }
@@ -325,7 +324,6 @@ export enum FeatureFlagKey {
   IsFunctionSettingsEnabled = 'IsFunctionSettingsEnabled',
   IsGmailSendEmailScopeEnabled = 'IsGmailSendEmailScopeEnabled',
   IsJsonFilterEnabled = 'IsJsonFilterEnabled',
-  IsMessageThreadSubscriberEnabled = 'IsMessageThreadSubscriberEnabled',
   IsMicrosoftSyncEnabled = 'IsMicrosoftSyncEnabled',
   IsPageHeaderV2Enabled = 'IsPageHeaderV2Enabled',
   IsPostgreSqlIntegrationEnabled = 'IsPostgreSQLIntegrationEnabled',