refactor: color/colors_gruvbox: Remove unused DEFAULT colors.

Author: neiljp

zulipterminal/config/color.py

@@ -37,7 +37,6 @@ class Background(Enum):
 # make use of the complete range instead of only 216 colors.
 class DefaultColor(Enum):
     # color       =  16code          256code    24code
-    DEFAULT       = 'default         default    default'
     BLACK         = 'black           g19        g19'
     DARK_RED      = 'dark_red        #a00       #a00'
     DARK_GREEN    = 'dark_green      #080       #080'

zulipterminal/themes/colors_gruvbox.py

@@ -21,7 +21,6 @@
 
 class GruvBoxColor(Enum):
     # color          =  16code          256code   24code
-    DEFAULT          = 'default         default   default'
 
     # Only or primarily dark mode - grayscales
     # - generally background