[pantsd] Remove dead code around prefork graph warming

[blorente]

Before removing forking from pantsd (#7596 ), we needed to warm up the v2 product graph of the daemon before forking into a pantsd-runner process (relevant code here).

Now, however, this warming is not necessary (because we don't fork a process per run of pants anymore), so we can do some cleanup to that code:

• We can remove the concept of "warming" (here), which is invoked from scheduler_service.prefork (here). This probably means that we can remove the entire warm_product_graph function.
• We can reword the code around scheduler_service.prefork so that it doesn't mention forking at all. It was called prefork because that's what we did before Pantsd without forking #7596, but it can be called something different now.
• We can merge the try:except block in DaemonPantsRunner.create() (here) into the try:except block in DaemonPantsRunner.run() (here). The run() function aims to centralize and correctly handle all the exceptions that happen on a pants run, so the "prefork" logic should be integrated there as well.

This will help a lot in correctly diagnosing pantsd problems, as this split and the wrangled exception handling that happen as a result have caused numerous issues.

[blorente]

Fixed by #8095