Remove cirq-rigetti module

[mhucka]

The Cirq Rigetti interface module was deprecated in Cirq version 1.5. Following the two-step process discussed in issue #7058, the cirq-rigetti module can be removed from the code base and documentation.

[mhucka]

Here's a proposed process for the removal:

1. Create a branch in the Cirq git repository with a meaningful name (perhaps cirq-rigetti-before-removal). This will act as a snapshot of the code base as it existed immediately before the module was removed.
2. Adjust the documentation files in Cirq docs/ as needed.
3. Adjust the Cirq GitHub CI workflows as needed.
4. Adjust the Cirq packaging and PyPI distribution workflows as needed.
5. Update quantumai.google/cirq documentation pages as needed.
6. Remove the module code from the Cirq code base and adjust the rest of Cirq as needed.