offlineimap: remove python prefix from package name

OfflineIMAP is primarily a program/tool, not a python module (although
it installs a python module too, for those who want to poke at its
internals).

Now we can install it with "nix-env -i offlineimap" instead of
"nix-env -i python2.7-offlineimap".

pkgs/tools/networking/offlineimap/default.nix

@@ -3,6 +3,7 @@
 buildPythonPackage rec {
   version = "6.5.5-rc2";
   name = "offlineimap-${version}";
+  namePrefix = "";
 
   src = fetchurl {
     url = "https://github.com/OfflineIMAP/offlineimap/tarball/v${version}";