[IMP] mrp: no help in workorder tablet view

For the `mrp.routing.workcenter`, removes the help from the `worksheet`
field to set it into the view.
This way, the tooltip doesn't appear each time the user cursor is placed
in front of the pdf in the workorder tablet view (`mpr_workorder`).

task-2225362

closes odoo/odoo#48903

Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>