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.

test/fuzz/fuzz-network-parser/directives

@@ -132,7 +132,6 @@ RouteMTUBytes=
 FallbackLeaseLifetimeSec=
 Use6RD=
 NetLabel=
-NFTSet=
 [DHCPv6]
 UseAddress=
 UseDelegatedPrefix=
@@ -155,7 +154,6 @@ IAID=
 DUIDType=
 DUIDRawData=
 NetLabel=
-NFTSet=
 [DHCPv6PrefixDelegation]
 SubnetId=
 Announce=
@@ -173,7 +171,6 @@ ManageTemporaryAddress=
 Token=
 RouteMetric=
 NetLabel=
-NFTSet=
 [Route]
 Destination=
 Protocol=
@@ -260,8 +257,6 @@ DHCPv6PrefixDelegation=
 DHCPPrefixDelegation=
 BatmanAdvanced=
 IPoIB=
-IPv4NFTSet=
-IPv6NFTSet=
 [IPv6Prefix]
 Prefix=
 OnLink=
@@ -353,7 +348,6 @@ Managed=
 OtherInformation=
 UplinkInterface=
 NetLabel=
-NFTSet=
 [IPv6PrefixDelegation]
 RouterPreference=
 DNSLifetimeSec=