Remove usage_data_i_source_code_code_intelligence feature flag
This is a flag that is used only for tracking purposes, and should be removed. Unfortunately, the current implementation of Gitlab::Utils::UsageData#track_usage_event
requires a feature flag to exist in perpetuity. This is being worked on in !48469 (closed), but until that launches, we're either blocked on removing the feature flag, or we need to modify the way we track this, and instead of Gitlab::Utils::UsageData#track_usage_event
use a wrapped counter method (as we do in Gitlab::UsageDataCounters::MergeRequestActivityUniqueCounter
)