feat: add support to double XPRV

[Tommytrg]

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

[parodyBit]

while we are moving the business logic related to the XPRV length to the witnet.dart library->
a double xprv has a unique path:
the wallet path is just m/0/0 and m/1/0 instead of m/3h/4919h/0h/0/0 and m/3h/4919h/0h/1/0 for external index 0 and internal index 0.
so it requires a unique sync routine and way to generate addresses for the custom path.
in which we have a unique xprv for both external and internal instead of being derived from a single xprv-

[gabaldon]

No need of this feature anymore