memory optimization: JBM MD handle allocated in non-JBM EXT operation

The JBM metadata (MD) handle st_ivas->hJbmMetadata is allocated also in non-JBM operation in EXT output which is useless.

A fix consists of avoiding its allocation in non-JBM scenarios which saves 3094 bytes in ISM / OMASA / OSBA formats when outputting to the EXT output config.