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.h

@@ -146,5 +146,3 @@ int parse_oom_score_adjust(const char *s, int *ret);
  * to a loadavg_t. */
 int store_loadavg_fixed_point(unsigned long i, unsigned long f, loadavg_t *ret);
 int parse_loadavg_fixed_point(const char *s, loadavg_t *ret);
-
-bool nft_identifier_bad(const char *id);