[Docs] Remove redundant note about cirq pre install in noise guide (#…

…4413)

Super small: Just noticed there are two identical notes about `pip install --pre cirq` in https://quantumai.google/cirq/noise

![Screenshot](https://user-images.githubusercontent.com/32416820/129090756-d8161156-e6b0-47d3-b1f4-9125d5b7ce54.png)

This removes one of them.

(I checked around other notebooks which do the --pre install and didn't find any other duplicates.)

docs/noise.ipynb

@@ -93,8 +93,6 @@
     "id": "7d3daa944d6b"
    },
    "source": [
-    "Note: this notebook relies on unreleased Cirq features. If you want to try these features, make sure you install cirq via `pip install cirq --pre`.\n",
-    "\n",
     "For simulation, it is useful to have `Gate` objects that enact noisy quantum evolution. Cirq supports modeling noise via *operator sum* representations of noise (these evolutions are also known as quantum operations or quantum dynamical maps). \n",
     "\n",
     "This formalism models evolution of the density matrix $\\rho$ via\n",