remove changelog key from Cargo.toml

The cargo feature (rust-lang/rfcs#2129) was never implemented, and this key
causes a cargo warning.

Author: vincentdephily

Cargo.toml

@@ -8,7 +8,6 @@ categories = ["development-tools::debugging"]
 license = "MIT/Apache-2.0"
 name = "stderrlog"
 readme = "README.md"
-changelog = "CHANGELOG.md"
 repository = "https://github.com/cardoe/stderrlog-rs"
 version = "0.4.4-alpha.0"