Issue #2283877 by dcam, larowlan: Remove or use Aggregator's teaser_length setting

aggregator.install

config/install/aggregator.settings.yml

config/schema/aggregator.schema.yml

migrations/d6_aggregator_settings.yml

migrations/d7_aggregator_settings.yml

src/Plugin/aggregator/processor/DefaultProcessor.php

tests/src/Functional/AggregatorAdminTest.php

.../Update/AggregatorUpdateDeleteTeaserLengthSettingTest.php

tests/src/Kernel/Migrate/d6/MigrateAggregatorConfigsTest.php

...s/src/Kernel/Migrate/d7/MigrateAggregatorSettingsTest.php