feat: add support to double XPRV

We are moving the business logic related to the XPRV length to the witnet.dart
library (https://github.com/witnet/witnet.dart). So, in this PR, we only remove
the XPRV length validation while importing a wallet. The logic to handle a
double XPRV is implemented here: witnet/witnet.dart#31

lib/screens/create_wallet/bloc/api_create_wallet.dart

@@ -50,8 +50,7 @@ class ApiCreateWallet {
     try {
       int xprvLength = xprvString.length;
       String? xprvDecripted;
-      if (xprvLength == ENCRYPTED_XPRV_LENGTH &&
-          _xprvType == CreateWalletType.encryptedXprv) {
+      if (_xprvType == CreateWalletType.encryptedXprv) {
         xprvDecripted = await apiCrypto.decryptXprv(
             xprv: xprvString, password: password?.value ?? '');
       } else if (xprvLength == XPRV_LENGTH &&