First off you should have sprung for the indirect in the first place because not only will it give you all the hot water you will need, it will also cut your oil bill by 15% That said, the restrictor is there to limit the flow through the coil. Coils are rated in gallons per minute at x degree of rise depending on boiler temperature. Without the restrictor your water would run cold quite often.
On a separate note. Tankless coils do not meet plumbing code. They have never met plumbing code but most inspectors have overlooked that little item for years, believing that folks could not afford to do anything different. Do yourself a huge favor and get rid of the thing.