config.fish: don't load default theme in noninteractive shells

We define colors in noninteractive shells for historical reasons
(because colors used to be universal variables).

The other potential reason is to get regular syntax highlighting for
commands like:

	fish -c 'read --shell'

but if anyone actually uses that they can probably load a theme
explicitly.

Stop defining colors in noninteractive shells.  It's usually not
a good idea to make them behave differently from interactive ones,
but color seems only relevant for interactive shells?

Let's see if anyone complains.. we may end up reverting this if people
want to use noninteractive fish to query colors..  but I'm not sure
why that would be necessary.

Closes #12673

tests/checks/__fish_migrate.fish

@@ -78,6 +78,7 @@ echo no default universal variables
 # Labeled color variables may be updated by fish.
 {
     $fish -c '
+        fish_config theme choose default
         set -g fish_color_autosuggestion red
         set -g fish_color_command green --theme=default
         __fish_migrate

tests/checks/fish_config.fish

@@ -1,6 +1,8 @@
 # RUN: %fish %s
 # REQUIRES: command -v diff
 
+fish_config theme choose default
+
 set -g fish (status fish-path)
 
 fish_config prompt list | string match -r '^(?:acidhub|disco|nim)$'

tests/checks/functions.fish

@@ -299,6 +299,7 @@ functions --no-details --color=never test_color_option
 # CHECK:     echo hello
 # CHECK: end
 
+fish_config theme choose default
 string escape (functions --no-details --color=always test_color_option)
 # CHECK: function\ \e\[36mtest_color_option\e\[32m
 # CHECK: \e\[39m\ \ \ \ echo\ \e\[36mhello\e\[32m