Remove Code Climate badge from the README

Code Climate score went from the max 4.0 to a lousy 1.0 for no
reason, when no changes were committed to the project.

I don't like to remove transparency like this, but this could be seen
as project quality decreasing, while there was nothing changed.
I am semi-temporarily removing this until I take the time to
investigate on what happened (if I ever do so, life is short).

I also became frustrated with Code Climate deciding to use their ESLint
configuration for linting while this project was using another
configuration (ant not using ESLint!). Again, maybe I'll re-add later,
if I think such metric is actually relevant (it did find some unused
variables and missing semicolons, so I would like to add it back).

README.md

@@ -3,7 +3,6 @@
 [![Build Status](https://travis-ci.org/astorije/chai-immutable.svg?branch=master)](https://travis-ci.org/astorije/chai-immutable)
 [![Build Status](https://ci.appveyor.com/api/projects/status/407ts84pq7wd4kt9/branch/master?svg=true)](https://ci.appveyor.com/project/astorije/chai-immutable/branch/master)
 [![Coverage Status](https://coveralls.io/repos/astorije/chai-immutable/badge.svg)](https://coveralls.io/r/astorije/chai-immutable)
-[![Code Climate](https://codeclimate.com/github/astorije/chai-immutable/badges/gpa.svg)](https://codeclimate.com/github/astorije/chai-immutable)
 [![devDependency Status](https://david-dm.org/astorije/chai-immutable/dev-status.svg)](https://david-dm.org/astorije/chai-immutable#info=devDependencies)
 [![peerDependency Status](https://david-dm.org/astorije/chai-immutable/peer-status.svg)](https://david-dm.org/astorije/chai-immutable#info=peerDependencies)