[IMP] membership: work on duration, not on date

Task: https://www.odoo.com/web#id=6806&view_type=form&model=project.task&action=333&active_id=131
Pad: https://pad.odoo.com/p/r.yIMRe1VVusFuPpnV
can you remove those 2 lines, your code should be editor agnostic! (use your .vimrc file if you want to customize the display of xml files)