Deletes default allowed_log_sources in statsd.

We should fail right away if someone forgets to set this field
instead of setting default values since this may lead to hard to
explain bugs in the future (eg, why isn't systemui logging an atom
to a config).

Bug: 74608359
Test: Verified unit-tests still pass on marlin-eng.
Change-Id: Ibd8a6ccbc5cc8d2dfb8a1577c64bf9b49822a2c3