Validated inventory adjustments create 'done' stock.move.line records, which cannot be deleted, hence the source of those stock.move.line records (the adjustment) cannot be deleted. For traceability purposes and to ensure that Odoo can properly account for what stock and product moves resulted in you having the current QoH, the way you would correct your inventory adjustment is via a new inventory adjustment. 

Also, you can export your inventory adjustment and simply reverse everything to save time.

The best thing is to reverse the inventory you just validated because Odoo is meant to keep the stock.move.line record intact and any way to remove it, would may cause you a problem with other parts of your system.

What I suggest is either you go to your old inventory. Export it and reverse the quantities then import it again, or id the inventory is small you can just go though each product template and adjust old quantity from the quantity on hand tab.