QCS Getting Started Guide - setup cleanup

[verult]

• remove default processor id
• set project env var

Note: get_qcs_objects_for_notebook() already selects a default processor id for device and sampler generation using the same logic, but the ID of the newly selected processor is not returned.

Alternatively, get_qcs_objects_for_notebook() can be modified to return a processor instead of a device and sampler, and retrieve the selected processor via processor_id. I don't have a preference between the two options.

There are 3 other spots where the processor id is hard-coded. If this approach looks good, I'll go ahead and update the other places, too.

cc @dstrain115 @wcourtney

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[verult]

This PR is now stale - get_qcs_objects_for_notebook() has been updated to use the latest form, and device.qubits has been fixed.

Will add logic to check processor health when picking a default processor in a separate PR.