task: remove ide_task_run_in_thread_sync()

This wasn't exactly able to handle all situations, so we removed
our use of it in Builder (which is better anyway). So now remove
this so we don't accidentally start relying on it in the future.
1 job for master in 23 minutes and 28 seconds (queued for 11 seconds)
Status Job ID Name Coverage
  Test
passed #16551
flatpak:master

00:23:28