strv: drop strv_join_quoted() (#8057)

The function `strv_join_quoted()` is now not used, and has a bug
in the buffer size calculation when the strings needs to escaped,
as reported in #8056.
So, let's remove the function.

Closes #8056.

src/basic/strv.h

@@ -86,7 +86,6 @@ char **strv_split_newlines(const char *s);
 int strv_split_extract(char ***t, const char *s, const char *separators, ExtractFlags flags);
 
 char *strv_join(char **l, const char *separator);
-char *strv_join_quoted(char **l);
 
 char **strv_parse_nulstr(const char *s, size_t l);
 char **strv_split_nulstr(const char *s);