Drop CHANGELOG.md (#1805)

Because that file was not kept up to for 4.0.1, 4.0.2, 4.1.0, 4.1.1, 5.0
.0, 5.0.1, 5.0.2, 5.0.3 and 5.0.4. Instead, we will keep using GitHub
auto-generated release notes.

As discussed in #1801 (comment)