net: remove CService::ToString() use ToStringAddrPort() instead

Both methods do the same thing, so simplify to having just one.

`ToString()` is too generic in this case and it is unclear what it does,
given that there are similar methods:
`ToStringAddr()` (inherited from `CNetAddr`),
`ToStringPort()` and
`ToStringAddrPort()`.

src/test/netbase_tests.cpp

@@ -131,7 +131,7 @@ BOOST_AUTO_TEST_CASE(netbase_splithost)
 bool static TestParse(std::string src, std::string canon)
 {
     CService addr(LookupNumeric(src, 65535));
-    return canon == addr.ToString();
+    return canon == addr.ToStringAddrPort();
 }
 
 BOOST_AUTO_TEST_CASE(netbase_lookupnumeric)