matplotlib-inline: Don't depend on matplotlib -> smaller ipython closure

The lib `matplotlib-inline` is only a shim to integrate `matplotlib` in
ipython, but it doesn't actually require it.

If one wants to use the matplotlib integration, `matplotlib` need to be
installed separatedly, then calling `%matplotlib inline` at a ipython
prompt will work.

Not including `matplotlib` is closer to a non-Nix installation:
Installing ipython in a fresh venv also does not install `matplotlib`.

NOTE: ipython's closure is now ~160MB instead of ~400MB.
(including ~100MB for python itself)

pkgs/development/python-modules/matplotlib-inline/default.nix

@@ -1,5 +1,4 @@
 { lib, buildPythonPackage, fetchPypi
-, matplotlib
 , traitlets
 
 # tests
@@ -16,7 +15,6 @@ buildPythonPackage rec {
   };
 
   propagatedBuildInputs = [
-    matplotlib # not documented, but required
     traitlets
   ];