Remove Bristlecone and Foxtail deprecated XmonDevices.

[MichaelBroughton]

Before removing XmonDevice outright, remove child classes.

[mpharrigan]

This raises some (imo) important JSON compatibility questions. #4321

As written, this PR drops support for a cirq_type completely and deletes the json test data (including the "inward" files). How hard would it be to include a shim to load the old data into a new GridDevice?

[mpharrigan]

although I see only the "old" files actually stored the qubit info; the others just give the device name. I don't know where/when that change was made. edit: changed very early on in #2527 with otherwise-unrelated changes, sadly

[mpharrigan]

Proposal: keep the json resolver but have it load into like the string name of the device or something; issue a warning telling users that these named devices no longer exist as of cirq version 0.16

or

port the named devices to GridDevice so that they still load in a devices with qubits and such and leave them be, as they are not hurting anyone and may be in some json documents.

[MichaelBroughton]

Went with your proposal. PTAL.

[mpharrigan]

LGTM -- the json_inward files are left unchanged, so that's good