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/core/execute.h

@@ -18,7 +18,6 @@ typedef struct Manager Manager;
 #include "cpu-set-util.h"
 #include "exec-util.h"
 #include "fdset.h"
-#include "firewall-util.h"
 #include "list.h"
 #include "missing_resource.h"
 #include "namespace.h"
@@ -314,9 +313,6 @@ struct ExecContext {
         bool mount_apivfs;
 
         bool dynamic_user;
-        size_t n_dynamic_user_nft_set_contexts;
-        NFTSetContext *dynamic_user_nft_set_context;
-
         bool remove_ipc;
 
         bool memory_deny_write_execute;
@@ -526,5 +522,3 @@ const char* exec_resource_type_to_string(ExecDirectoryType i) _const_;
 ExecDirectoryType exec_resource_type_from_string(const char *s) _pure_;
 
 bool exec_needs_mount_namespace(const ExecContext *context, const ExecParameters *params, const ExecRuntime *runtime);
-
-void exec_delete_dynamic_user_nft_set(const ExecContext *c, DynamicUser *d);