Remove fetchMD5warn

Deprecation warnings should not be used in Nixpkgs because they spam
innocent "nix-env -qa" users with (in this case) dozens of messages
that they can't do anything about.

This also reverts commit 2ca8833383.

lib/trivial.nix

@@ -138,7 +138,4 @@ rec {
   */
   warn = msg: builtins.trace "WARNING: ${msg}";
   info = msg: builtins.trace "INFO: ${msg}";
-
-  fetchMD5warn = name: context : data : info
-    "Deprecated use of MD5 hash in ${name} to fetch ${context}" data;
 }