Remove serve handler from `build_runner`

[natebosch]

Replaced by webdev serve which has it's own hot reloading handling.

We should be able to remove:

• The serve command and all related code.
• All code related to hot reloading.

[evanweible-wf]

If/when the serve command is removed, will there still be a way to utilize --track-performance? Or is there another way to view the perf data after a build runs with that flag enabled?

[natebosch]

We'd implement some other mechanism for --track-performance.

[jakemac53]

It was always a bit weird that you had to be in serve mode for that to work, and it has never worked with webdev or any other build daemon client I don't think, so it needs a revisit anyways :).