Remove code_navigation_references feature flag

What

Remove the :code_navigation_references feature flag introduced in #217392 (closed) which was enabled by default in #225621 (closed).

Owners

• Team: groupsource code
• Most appropriate slack channel to reach out to: #g_create_source-code
• Best individual to reach out to: @patrickbajao

Expectations

What are we expecting to happen?

LSIF json files will still be generated for those projects with code intelligence enabled.

What might happen if this goes wrong?

Nothing since it's already been enabled by default.

What can we monitor to detect problems with this?

• workhorse API max ram usage
• workhorse API avg ram usage

Roll Out Steps

• Remove feature flag and add changelog entry
• After the flag removal is deployed, clean up the feature flag by running chatops command in #production channel