Fix deprecated implicit copy ctors in IP code

Fix some warnings present in GCC8/9 in the IPAddress code

In AddressListIterator there was a copy constructor which simply copied
the structure bit-for-bit.  That's the default operation, so remove it
to avoid the warning there.

IPAddress, add a default copy constructor since the other copy
constructors are simply parsing from one format into a native ip_addr_t.

@d-a-v, can you give these a look over and see if they're good (since
IP stuff is really your domain).

cores/esp8266/AddrList.h

@@ -167,8 +167,6 @@ class AddressListIterator
     bool operator== (AddressListIterator& o) { return netIf.equal(*o); }
     bool operator!= (AddressListIterator& o) { return !netIf.equal(*o); }
 
-    AddressListIterator& operator= (const AddressListIterator& o) { netIf = o.netIf; return *this; }
-
     AddressListIterator operator++ (int)
     {
         AddressListIterator ret = *this;

cores/esp8266/IPAddress.h

@@ -136,6 +136,7 @@ class IPAddress: public Printable {
         // Overloaded copy operators to allow initialisation of IPAddress objects from other types
         IPAddress& operator=(const uint8_t *address);
         IPAddress& operator=(uint32_t address);
+	IPAddress& operator=(const IPAddress&) = default;
 
         virtual size_t printTo(Print& p) const;
         String toString() const;