refactor: migrate ENTERPRISE_ENABLED off FEATURES-as-dict

The learner_home view tests carried seven
`@patch.dict(settings.FEATURES, ENTERPRISE_ENABLED=False)` decorators.
Note this is the kwarg form, so it set a FEATURES key literally named
'ENTERPRISE_ENABLED' — which nothing in the codebase reads (the enterprise
dashboard is gated by ENABLE_ENTERPRISE_INTEGRATION). The nearby module
constant `ENTERPRISE_ENABLED = "ENABLE_ENTERPRISE_INTEGRATION"` was never
referenced by these kwargs, so the override has always been a no-op; it
looks like the author intended to disable ENABLE_ENTERPRISE_INTEGRATION.

This change is scoped to getting off the FEATURES dict, not fixing the
latent no-op: convert the decorators to the equivalent
`@override_settings(ENTERPRISE_ENABLED=False)` (behavior identical — still
sets an unread flat attribute) and drop the now-dead, misleading constant.
A follow-up could decide whether these tests meant to assert the
enterprise-disabled path via ENABLE_ENTERPRISE_INTEGRATION.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>