fix(account):if new account can't pay NewAccountTax, it must be removed

When an account was deleted, it was not really deleted, its balance was just set to zero.

So the account was still considered as existing from the point of view of the System::account_exists() function.

Closes #60 (closed)

pallets/duniter-account/src/lib.rs

@@ -198,13 +198,11 @@ pub mod pallet {
                             total_weight += 200_000;
                         }
                     } else {
-                        // The charges could not be deducted, we slash the account
+                        // The charges could not be deducted, we must destroy the account
                         let balance_to_suppr =
                             account_data.free.saturating_add(account_data.reserved);
                         // Force account data supression
-                        frame_system::Account::<T>::mutate(&account_id, |a| {
-                            a.data.set_balances(Default::default())
-                        });
+                        frame_system::Account::<T>::remove(&account_id);
                         Self::deposit_event(Event::ForceDestroy {
                             who: account_id,
                             balance: balance_to_suppr,