Remove prefix hardcoding in validate_target()

[bharat-thotakura]

Addresses part of #958 by unblocking the backends() endpoint of qiskit-superstaq.

Will also allow calls to the compile endpoint with iqm_* backends (though further optimization may be needed).

[richrines1]

lgtm! though we should think about removing the hard-coded vendors entirely..

[bharat-thotakura]

^ I can go ahead & remove the following:

client-superstaq/general-superstaq/general_superstaq/validation.py

Lines 66 to 71 in 464e900

	# Check valid prefix
	if prefix not in vendor_prefixes:
	raise ValueError(
	f"{target!r} does not have a valid target prefix. Valid prefixes are: "
	f"{vendor_prefixes}."
	)

I just wasn't sure what the original motivation was to have that check or what other condition we should replace it with, if any

[richrines1]

i think it's fine to remove. i believe the motivation was just to catch easy errors before they hit the server, but now that our list of supported devices is somewhat dynamic this doesn't make much sense