fish_config prompt save: don't define right prompt unnecessarily

If no "fish_right_prompt" is defined, then we also don't need to
define and save an empty one.  We can do nothing, which seems cleaner.
It's also what "fish_config prompt choose" does.  Git blame shows no
explicit reason for for this inconsistency.

Probably the implicit reason is that when both styles where introduced
(both in 69074c1591 (fish_config: Remove right prompt in `choose` and
`save`, 2021-09-26)), that was before the "fish_config prompt choose:
make right prompt hack less weird" commit, so the "prompt choose"
code path didn't know whether the prompt existed until after checking,
hence "--erase" was more suitable.. but that inconsistency is gone.

share/functions/fish_config.fish

@@ -130,7 +130,11 @@ function fish_config --description "Launch fish's web based configuration"
                     funcsave fish_prompt
                     or return
 
-                    funcsave fish_right_prompt 2>/dev/null
+                    if functions -q fish_right_prompt
+                        funcsave fish_right_prompt
+                    else
+                        rm -f $__fish_config_dir/functions/fish_right_prompt.fish
+                    end
                     return
             end
 
@@ -362,7 +366,8 @@ function __fish_config_prompt_save
     # if necessary.
     function fish_prompt
     end
-    function fish_right_prompt
+    if functions -q fish_right_prompt
+        functions --erase fish_right_prompt
     end
     source $argv[1] # N.B We're passed either stdin or argv.
 end