A code P Form 1099-R would only affect your tax return if box 2a reported a nonzero amount of earnings that were taxable and potentially subject to early-distribution penalty on your 2017 tax return. So if box 2a has a nonzero amount that was not previously included in your taxable income you should amend, otherwise it's probably not necessary to amend.
Your 2017 tax return should have included an explanation of the original contribution and the return of that contribution, otherwise the IRS might question the fact that the financial institution issued a Form 5498 reporting the original contribution that included the excess and your tax return did not show the excess contribution on Form 5329.
