Properly unregister gpgex when uninstalling 32-bit installations

This mimics what's done when uninstalling gpgol. Moreover, don't try to
uninstall $INSTDIR\bin_64\gpgex.dll for 64-bit installations where this
file doesn't exist.

These changes complement the changes that were done for inst-gpgex.nsi
with 1ca8e131fc69e0a460ee0a97e340cd6568e6c8a7.