It is a requirement of EC Council Regualtion 3922/91 that the national aviation authorities of EC member states should be members of the JAA. It would therefore not be legal under current EC legislation for Germany to withdraw from the JAA.
However, a new Regulation is being currently going through the EC legislatiove process to establish the European Aviation Safety Agency (EASA). If the Regulation is adopted (it has passed the first reading)EASA will replace the JAA as far as as EC (and EEA?) countries are concerned.