Respect umask settings in Transmission config

In NixOS/nixpkgs@da6bc44 @thoughtpolice
made the Transmission NixOS module override the umask setting in the
Transmission config. This commit removes that override.

I want a different umask setting and I guess it is possible that other
people might want it to. Thus I think it is a good idea to respect the
umask settings in the Transmission config.