fish: Use native hostname variable

$hostname is now defined by default so we don't need to do anything
here. I assume it wasn't always like this since otherwise I wouldn't
have needed to call `hostname` in the first place.

fish/.config/fish/functions/fish_prompt.fish

@@ -5,8 +5,6 @@ function fish_prompt
 
     # Use a more informative and non-unicode prompt for ttys
     if status --is-login; and test -z "$DISPLAY"
-        set hostname (hostname)
-
         set_color yellow;   echo -n "$USER@$hostname"
         set_color normal;   echo -n " "
         set_color magenta;  echo -n "($PWD)"