Remove invalid setting

When there's no picture, this does not make sense.

dotfiles/config/xfce4/terminal/terminalrc

 MiscTabCloseMiddleClick=TRUE
 MiscTabPosition=GTK_POS_TOP
 MiscHighlightUrls=TRUE
-BackgroundMode=TERMINAL_BACKGROUND_IMAGE
 BackgroundDarkness=0.950000
 TitleMode=TERMINAL_TITLE_REPLACE
 MiscBellUrgent=TRUE