Remove track_application_boot_time feature flag

What does this MR do and why?

This drops the track_application_boot_time feature flag. We had deployed this with default: on originally (see discussion in !79139 (merged) as to why).

So this has been running for over a week now without any known problems and should be safe to remove.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #351769 (closed)