[FIX] l10n_be Partner VAT Intra XML delaration: Vat number format correction

The Partner VAT Intra XML delaration for Belgium needs to have the VAT number
in the format without the country code prefix.

The current behaviour was to include the country code prefix in the
report which causes an "SQLException in IbatisBtwSignaleticDaoImpl::search" error upon 
uploading the XML file to the Federal goverment's website.  While the error is non-descriptive
fixing the VAT number format fixes the error.

Below a screenshot of the error (in dutch):
![image](https://cloud.githubusercontent.com/assets/7703788/3363188/7f58bae4-fb11-11e3-8fa2-6d5686ef07ab.png)