This one was due to previous design https://www.dropbox.com/sh/8jkb0osnvfda96e/mAsJQHfWsi/_Old/_old%20visuals#f:08%20SMS_Edit.png

However, following the current design 
https://www.dropbox.com/sh/8jkb0osnvfda96e/TkcQuTbhHj#f:02.SMS.MMS_0002_Thread_EditMode.png

So we could remove the line!