Message export: Fix job progress calculation (#21925)

* use posts count instead of job duration to calculate progress

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

i18n/en.json

@@ -7571,14 +7571,6 @@
     "id": "ent.id_loaded.license_disable.app_error",
     "translation": "Your license does not support ID Loaded Push Notifications."
   },
-  {
-    "id": "ent.jobs.do_job.batch_size.parse_error",
-    "translation": "Could not parse message export job BatchSize."
-  },
-  {
-    "id": "ent.jobs.do_job.batch_start_timestamp.parse_error",
-    "translation": "Could not parse message export job ExportFromTimestamp."
-  },
   {
     "id": "ent.jobs.start_synchronize_job.timeout",
     "translation": "Reached AD/LDAP synchronization job timeout."