Remove the changelog file (#414)

Since #396 ignis has moved to a model where the changelog is
automatically generated from the git log and github labels at release
time and using reno for release documentation [1][2]. That commit was supposed
to remove the changelog file so people did not go to use it anymore
since it's not needed (and all the information is duplicated with the
git log and release pages). This commit corrects the oversight and
removes the unused file to avoid confusion.

[1] https://github.com/Qiskit/qiskit-ignis/blob/master/CONTRIBUTING.md#changelog-generation
[2] https://github.com/Qiskit/qiskit-ignis/blob/master/CONTRIBUTING.md#release-notes