Don't allow the target to decline a share

[mgiuca]

The current spec has a provision for the target to "decline to receive the data". Despite this, neither the existing Android implementation on Chrome nor the proposed Web Share Target URL-template-based API would allow the target to respond with a failure state.

Given that we aren't likely to make use of this in the near future, and that it could be re-added in a backwards-compatible way if we wanted to introduce a new failure mode, let's just remove this text from the spec for now.