remove unnecesarry fallback_if_none functionality in custom groups

No longer needed with the new change in shader.

lua/mellifluous/highlights/custom_groups.lua

@@ -1,12 +1,5 @@
 local M = {}
 
-local function fallback_if_none(color, fallback_color)
-    if type(color) == "string" and color == "NONE" then
-        return fallback_color
-    end
-    return color
-end
-
 -- Any shared highlight groups that cannot be found in general highlights or
 -- treesitter highlights are created here.
 function M.get(colors)
@@ -17,11 +10,9 @@ function M.get(colors)
         MainKeyword = { fg = colors.main_keywords, style = config.styles.main_keywords or {} },
 
         IndentLine = function(bg)
-            bg = fallback_if_none(bg, colors.bg)
             return { fg = shader.replicate_shade(colors.bg, colors.fg5, bg) }
         end,
         IndentLineInactive = function(bg)
-            bg = fallback_if_none(bg, colors.bg)
             return {
                 fg = config.is_bg_dark and shader.replicate_shade(colors.bg, colors.bg4, bg)
                     or shader.replicate_shade(colors.bg, colors.dark_bg2, bg),
@@ -30,7 +21,6 @@ function M.get(colors)
 
         MenuButton = { fg = colors.ui_blue },
         MenuButtonSelected = function(bg)
-            bg = fallback_if_none(bg, colors.bg)
             local applied_bg = config.is_bg_dark and shader.replicate_shade(colors.bg, colors.bg4, bg)
                 or shader.replicate_shade(colors.bg, colors.dark_bg2, bg)
             return {