complete: Stop using the function definition as the description

This isn't helpful, and entirely unreadable. Excerpt:

```
__fish_git_prompt_set_char  (set -l user_variable_name "$argv[1]" set -l char $argv[2] if set -q argv[3] and begin set -q __fish_git_prompt_show_informative_status or set -q __fi…)
```

Fixes #7911.

src/complete.cpp

@@ -713,14 +713,7 @@ void completer_t::complete_cmd_desc(const wcstring &str) {
 /// Returns a description for the specified function, or an empty string if none.
 static wcstring complete_function_desc(const wcstring &fn) {
     wcstring result;
-    bool has_description = function_get_desc(fn, result);
-    if (!has_description) {
-        function_get_definition(fn, result);
-        // A completion description is a single line.
-        for (wchar_t &c : result) {
-            if (c == L'\n') c = L' ';
-        }
-    }
+    function_get_desc(fn, result);
     return result;
 }