Fix gruvbox diff colors

See:
https://github.com/ellisonleao/gruvbox.nvim/issues/105#issuecomment-1480547367

lua/plugins/gruvbox.lua

@@ -1,5 +1,3 @@
-local colors = require("gruvbox.palette").colors;
-
 require("gruvbox").setup({
   undercurl = true,
   underline = true,
@@ -36,11 +34,3 @@ require("gruvbox").setup({
   }
 })
 vim.o.background = "dark"
-
--- TODO: how to apply this when colorscheme changes?
-vim.api.nvim_set_hl(0, 'DiffDelete', { bg = "none", fg = colors.dark2 })
-vim.api.nvim_set_hl(0, 'DiffviewDiffDelete', { bg = "none", fg = colors.dark2 })
-vim.api.nvim_set_hl(0, 'DiffviewDiffAddAsDelete', { bg = "#370000" })
-vim.api.nvim_set_hl(0, 'DiffAdd', { bg = "#393400" }) -- #122f2f
-vim.api.nvim_set_hl(0, 'DiffChange', { bg = "#222a36" })
-vim.api.nvim_set_hl(0, 'DiffText', { bg = "#2f3f5c" }) -- #2f3f5c