Revert NFTSet feature

This reverts PR #22587 and its follow-up commit. More specifically,
2299b1cae3 (partially),
e176f855278d5098d3fecc5aa24ba702147d42e0,
ceb46a31a01b3d3d1d6095d857e29ea214a2776b, and
51bb9076ab8c050bebb64db5035852385accda35.

The PR was merged without final approval, and has several issues:
- OSS fuzz reported issues in the conf parser,
- It calls synchrnous netlink call, it should not be especially in PID1,
- The importance of NFTSet for CGroup and DynamicUser may be
  questionable, at least, there was no justification PID1 should support
  it.
- For networkd, it should be implemented with Request object,
- There is no test for the feature.

Fixes #23711.
Fixes #23717.
Fixes #23719.
Fixes #23720.
Fixes #23721.
Fixes #23759.

src/basic/parse-util.c

@@ -750,38 +750,3 @@ int parse_loadavg_fixed_point(const char *s, loadavg_t *ret) {
 
         return store_loadavg_fixed_point(i, f, ret);
 }
-
-static bool nft_first_char_bad(const char c) {
-        if ((c >= 'a' && c <= 'z') ||
-            (c >= 'A' && c <= 'Z'))
-                return false;
-        return true;
-}
-
-static bool nft_next_char_bad(const char c) {
-        if ((c >= 'a' && c <= 'z') ||
-            (c >= 'A' && c <= 'Z') ||
-            (c >= '0' && c <= '9') ||
-            c == '/' || c == '\\' || c == '_' || c == '.')
-                return false;
-        return true;
-}
-
-/* Limitations are described in https://www.netfilter.org/projects/nftables/manpage.html and
- * https://bugzilla.netfilter.org/show_bug.cgi?id=1175 */
-bool nft_identifier_bad(const char *id) {
-        assert(id);
-
-        size_t len;
-        len = strlen(id);
-        if (len == 0 || len > 31)
-                return true;
-
-        if (nft_first_char_bad(id[0]))
-                return true;
-
-        for (size_t i = 1; i < len; i++)
-                if (nft_next_char_bad(id[i]))
-                        return true;
-        return false;
-}