Remove CPDFSDK_Annot::ResetAppearance().

It's unused, and it caused a warning about CPDFSDK_Widget::ResetAppearance()
failing to override it (since these two unrelated methods had the same name).

No intended behavior change.

BUG=pdfium:29
R=bo_xu@foxitsoftware.com

Review URL: https://codereview.chromium.org/429483004