Remove references to cirq_google.Foxtail (#6126)

Does not exist anymore.  Replace with cirq_google.Sycamore.

docs/dev/development.md

@@ -287,7 +287,7 @@ def some_method(a: int, b: str) -> float:
     Notice that this docstring is an r-string, since the latex has backslashes.
     We can also include example code:
 
-        print(cirq_google.Foxtail)
+        print(cirq_google.Sycamore) 
 
     You can also do inline latex like $y = x^2$ and inline code like
     `cirq.unitary(cirq.X)`.
@@ -415,6 +415,6 @@ python dev_tools/requirements/reqs.py dev_tools/requirements/dev.env.txt
 
     ```bash
     python -m pip install cirq
-    python -c "import cirq; print(cirq_google.Foxtail)"
+    python -c "import cirq; print(cirq_google.Sycamore)"
     python -c "import cirq; print(cirq.__version__)"
     ```

docs/google/devices.md

@@ -333,14 +333,3 @@ It can be accessing by using `cirq_google.Bristlecone`. Circuits can be compiled
 `cirq_google.optimized_for_xmon` or by using `cirq_google.optimized_for_sycamore` with
 optimizer_type `xmon`.
 
-### Foxtail
-
-The Foxtail device is a 2 by 11 XMON device arranged in a bilinear array,
-addressable by using grid qubits `({0,1}, {0-11})`. It was one of the first
-super-conducting quantum devices announced by Google. Due to the small number of qubits
-and limited connectivity, it is still interesting for exploring the space of constrained
-algorithms on NISQ devices.
-
-It can be accessing by using `cirq_google.Foxtail`. Circuits can be compiled to it by using
-`cirq_google.optimized_for_xmon` or by using `cirq_google.optimized_for_sycamore` with
-optimizer_type `xmon`.