[IMP] Membership: cleaning

addons/event/board_association_view.xml

addons/event/event_view.xml

addons/event/report/report_event_registration.py

addons/event/report/report_event_registration_view.xml

addons/membership/membership.py

addons/membership/membership_demo.xml

addons/membership/membership_view.xml

addons/membership/report/report_membership.py

addons/membership/report/report_membership_view.xml

addons/membership/wizard/membership_invoice.py

addons/membership/wizard/membership_invoice_view.xml

addons/membership/wizard/membership_unpaid_invoice.py

addons/membership/wizard/membership_unpaid_invoice_view.xml