[addrman] Remove all public uses of CAddrMan.Clear() from the tests

Just use unique_ptr<CAddrMan>s and reset the pointer if a frest addrman is required.
Also make CAddrMan::Clear() private to ensure that no call sites are missed.

src/bench/addrman.cpp

@@ -72,11 +72,9 @@ static void AddrManAdd(benchmark::Bench& bench)
 {
     CreateAddresses();
 
-    CAddrMan addrman(/* deterministic */ false, /* consistency_check_ratio */ 0);
-
     bench.run([&] {
+        CAddrMan addrman{/* deterministic */ false, /* consistency_check_ratio */ 0};
         AddAddressesToAddrMan(addrman);
-        addrman.Clear();
     });
 }