Remove HmrTarget (#97253)
With #94948 we intended to move the client over to the firehose feed of
HMR events with the intent of unifying the code paths for maintenance.
However, now that Server HMR is moving to a pull-based model (which
client HMR will not be able to implement), let's keep the split.
There's no need to encode the HmrTarget into each surface, and we can
just use the function name to indicate which mode of HMR it's for.
