Well, i succeed in removing passphrase or creating new key without passphrase by using pinentry-gtk-2 as the pinentry program.
Here is what i've done:
There is no config file in ~/.gnupg.
i created a symbolic link from /usr/bin/pinentry to /usr/bin/pinentry-gtk-2
sudo ln -s /usr/bin/pinentry-gtk-2 /usr/bin/pinentry
It also worked over an SSH connection with X11 forwarding.