Cleanup `explore_pipeline_status` feature flag

Context

The feature flag was introduced in Remove pipeline status from /explore (!51621 - merged) to address the performance problem described in https://gitlab.com/gitlab-org/gitlab/-/issues/260334+.

This feature flag is disabled on GitLab.com that means the pipeline status is not displayed on Explore page.

Proposal

Consider that pipeline status is not displayed for a while and there are no complaints about it, we can make this change permanent and don't show pipeline status on Explore page anymore.