Revert "add the manpath to the installer"

This reverts commit 76f985b92d. We
shouldn't mess with $MANPATH, because on some "man" implementations
(like NixOS'), the default value on $MANPATH is derived from $PATH. So
if you set $MANPATH, you lose the default locations.

scripts/nix-profile.sh.in

@@ -9,7 +9,6 @@ if [ -n "$HOME" ]; then
     fi
 
     export PATH=$NIX_LINK/bin:$NIX_LINK/sbin:$PATH
-    export MANPATH=$NIX_LINK/share/man:$MANPATH
 
     # Subscribe the user to the Nixpkgs channel by default.
     if [ ! -e $HOME/.nix-channels ]; then