Trying to delete a line without messing the face

Hello. I have been working on a project where i had to plot land on landscape. I had originally finished but now have to make changes to the land layout. I deleted the plot but im having problems deleting a line without it deleting the face.

The line in the red circle is what im trying to delete. Any ideas? Attaching the sketchup file as well. Many thanks
land.skp (8.5 MB)

You just need to soften it, here I have use ctrl with the eraser tool.

Been trying to figure this out for hours. Thank you so much!