Remove libextra dependency

Cargo.toml

@@ -3,8 +3,5 @@ name = "ralloc"
 version = "0.1.0"
 authors = ["ticki <ticki@users.noreply.github.com>"]
 
-[dependencies.extra]
-git = "https://github.com/redox-os/libextra.git"
-
 [target.'cfg(unix)'.dependencies]
 syscall = "0.2.1"