[Feature flag] Remove import graceful failures feature flag

What

In #37057 (closed) the feature was tested in production and enabled by default. This issue removes the :import_graceful_failures feature flag

Owners

• Team: memory team
• Most appropriate slack channel to reach out to: #g_memory
• Best individual to reach out to: @nmilojevic1

Based on documentation, we should clean up feature flag, once this is deployed:

• /chatops run feature delete import_graceful_failures --staging
• /chatops run feature delete import_graceful_failures