[FIX] osv.fields: avoid unbinding _symbol_* of fields 
[FIX] osv.fields: avoid unbinding _symbol_* of fields 

Reverts rev-id: odo@openerp.com-20110112132828-a38e8ow7ahu97jy3
vnc-biz/openerp-server,gisce/openobject-server,splbio/openobject-server,vnc-biz/openerp-server,MarkusTeufelberger/openobject-server,splbio/openobject-server,ovnicraft/openerp-server,splbio/openobject-server,MarkusTeufelberger/openobject-server,MarkusTeufelberger/openobject-server,ovnicraft/openerp-server,gisce/openobject-server,gisce/openobject-server