fish_config: Remove superfluous grep

`grep -Fx` will match fixed strings over the entire line.

`status list-files foo` will match anything that starts with "foo".

Since we always pass a suffix here, that's the same thing.
If we really cared, we could do `string match "*.$suffix"` or similar.

Fixes #12060

share/functions/fish_config.fish

@@ -376,7 +376,7 @@ function __fish_config_matching
         if not set -q argv[1]
             set -a paths (status list-files $prefix)
         else
-            set -a paths (status list-files $prefix | grep -Fx -e"$prefix/"$argv$suffix)
+            set -a paths (status list-files "$prefix/"$argv$suffix)
         end
     end
     string join \n $paths