Remove ci_enable_live_trace feature flag

Description

After we evaluated new live trace feature on gitlab.com, we concluded that this feature doesn't have a threat even if it's enabled on gitlab.com scale. Thus, we can safely remove this feature flag to make this feature always ON.

Proposal

• Remove ci_enable_live_trace feature flag
• Update documents
  • https://docs.gitlab.com/ee/administration/object_storage.html#incremental-logging-is-required-for-ci-to-use-object-storage
  • https://docs.gitlab.com/ee/administration/job_artifacts.html#using-object-storage
  • https://docs.gitlab.com/ee/administration/job_logs.html#new-incremental-logging-architecture

Blockers

• If object storage is not enabled, we can't enable this FF by default. Until we resolve that problem, this feature should not be available out of the box.

/cc @ayufan @erushton