fix: don't send x-mc-account header for tenant-specific credentials

getMimecastClientForTenant now always uses accountCode='' so the
x-mc-account header is never sent. Sending your own account code
with tenant-specific OAuth credentials causes Mimecast to 403
(it interprets it as an invalid impersonation attempt).

lib/services/mimecast-client.ts

@@ -569,12 +569,11 @@ export function getMimecastClientForTenant(tenant: {
   client_id: string;
   client_secret: string;
   base_url?: string;
-  account_code?: string;
 }): MimecastClient {
   return new MimecastClient({
     clientId: tenant.client_id,
     clientSecret: tenant.client_secret,
     baseUrl: tenant.base_url ?? 'https://api.services.mimecast.com',
-    accountCode: tenant.account_code ?? '',
+    accountCode: '',
   });
 }