Revert "networkd: NetLabel integration"

This reverts PR #23269 and its follow-up commit. Especially,
2299b1cae3 (partially), and
3cf63830ac.

The PR was merged without final approval, and has several issues:
- The NetLabel for static addresses are not assigned, as labels are
  stored in the Address objects managed by Network, instead of Link.
- If NetLabel is specified for a static address, then the address
  section will be invalid and the address will not be configured,
- It should be implemented with Request object,
- There is no test about the feature.

test/fuzz/fuzz-network-parser/directives

@@ -131,7 +131,6 @@ MUDURL=
 RouteMTUBytes=
 FallbackLeaseLifetimeSec=
 Use6RD=
-NetLabel=
 [DHCPv6]
 UseAddress=
 UseDelegatedPrefix=
@@ -153,7 +152,6 @@ RouteMetric=
 IAID=
 DUIDType=
 DUIDRawData=
-NetLabel=
 [DHCPv6PrefixDelegation]
 SubnetId=
 Announce=
@@ -161,7 +159,6 @@ Assign=
 ManageTemporaryAddress=
 Token=
 RouteMetric=
-NetLabel=
 [DHCPPrefixDelegation]
 UplinkInterface=
 SubnetId=
@@ -170,7 +167,6 @@ Assign=
 ManageTemporaryAddress=
 Token=
 RouteMetric=
-NetLabel=
 [Route]
 Destination=
 Protocol=
@@ -347,7 +343,6 @@ EmitDomains=
 Managed=
 OtherInformation=
 UplinkInterface=
-NetLabel=
 [IPv6PrefixDelegation]
 RouterPreference=
 DNSLifetimeSec=