Move random circuits to ReCirq

[dabacon]

The circuits in
generate_boixo_2018_supremacy_circuits_v2.py
feel much more like they belong in ReCirq than in cirq.google

[balopat]

Agreed!

[mpharrigan]

I think this refers to cirq.experiments.google_v2_supremacy_circuit rather than the above named file in cirq.google.

[mpharrigan]

xref #2235

[mpharrigan]

xref #3775

[dstrain115]

This doesn't sound too difficult, and between Matt and I, we should move this to ReCirq before 1.0