Remove plotting (#232)

* Remove reference to obsolete plotting.ipynb

plotting.ipynb was removed in #115.

* add line about plotting gallery

doc/source/tutorials/pyam_first_steps.ipynb

@@ -379,8 +379,7 @@
    "source": [
     "## Visualization of timeseries\n",
     "\n",
-    "This section provides one illustrative example of the plotting features of the ``pyam`` package.  \n",
-    "Please see the `plotting.ipynb` notebook for a full tutorial on plotting."
+    "This section provides one illustrative example of the plotting features of the ``pyam`` package. Please see the [plotting gallery](https://pyam-iamc.readthedocs.io/en/latest/examples/index.html) for more examples on plotting with `pyam`."
    ]
   },
   {
@@ -651,7 +650,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.6.7"
+   "version": "3.7.3"
   }
  },
  "nbformat": 4,