Remove DIGEST_IMMEDIATELY option

Although DIGEST_IMMEDIATELY=True is theoretically a nice thing to
have, the upkeep is not worth it now that we're about to introduce
minidump ingestion.

The only thing that you're saving is the round-trip via the filesystem,
but performance of that is negligable, and if you're configuring
DIGEST_IMMEDIATELY you're actually _not_ in the performance-critical path
anyway.

Getting rid of it _also_ harmonizes/reduces the number of paths to test.

It's approximately 1% of our installed base.

phonehome/tasks.py

@@ -92,7 +92,6 @@ def _make_message_body():
             "SINGLE_TEAM": get_settings().SINGLE_TEAM,
             "EMAIL_BACKEND": settings.EMAIL_BACKEND,
             "TASK_ALWAYS_EAGER": get_snappea_settings().TASK_ALWAYS_EAGER,
-            "DIGEST_IMMEDIATELY": get_settings().DIGEST_IMMEDIATELY,
             "IS_DOCKER": settings.IS_DOCKER,
             "DATABASE_ENGINE": settings.DATABASES["default"]["ENGINE"],
         },