Follow-up from "Fix lint issues in internal/senddata"

The following discussion from !151137 (merged) should be addressed:

• @ashmckenzie started a discussion: (+2 comments)

  suggestion: I don't think we actually need the contentDisposition.Flush() method as I can't see any other calls to it and it calls contentDisposition.FlushError() anyway (and doesn't return it's error).

  Once contentDisposition.Flush() is removed, we'll also need to update workhorse/internal/senddata/contentprocessor/contentprocessor.go:33 to be:

  		defer func() { _ = cd.FlushError() }()

  While we're still throwing away the error here, I feel it's more obvious why we're doing that compared to before (presumably because we don't care).

  @ck3g as someone who has worked on this code in the past, may you please provide your thoughts on my understanding/suggestion, thanks 🙇