Do not allow overriding fish_term24bit

Considering this tool is not related to the fish shell itself, there is
no good reason to make this configurable elsewhere.

saury

@@ -193,8 +193,7 @@ function on_start
             or test "$COLORTERM" = truecolor -o "$COLORTERM" = 24bit
         end
 
-        set -q fish_term24bit
-        or set -g fish_term24bit 1
+        set -g fish_term24bit 1
     end
 
     if not test -t 1