overlays: remove 'gruvbox-nvim-better-diff'

Now that I have the explicit override for it, this overlay is not
necessary.

This reverts commit 9e0930aca4.
This reverts commit 28187c3b8f.

overlays/gruvbox-nvim-better-diff/colours.patch

@@ -1,28 +0,0 @@
-From 416b3c9c5e783d173ac0fd5310a76c1b144b92c1 Mon Sep 17 00:00:00 2001
-From: eeeXun <sdes96303@gmail.com>
-Date: Thu, 19 Oct 2023 02:34:12 +0800
-Subject: Use better diff colours
-
----
- README.md       | 3 ++-
- lua/gruvbox.lua | 7 ++++---
- 2 files changed, 6 insertions(+), 4 deletions(-)
-
-diff --git a/lua/gruvbox.lua b/lua/gruvbox.lua
-index ceba0735..a319fc6a 100644
---- a/lua/gruvbox.lua
-+++ b/lua/gruvbox.lua
-@@ -360,9 +361,9 @@ local function get_groups()
-     PmenuSel = { fg = colors.bg2, bg = colors.blue, bold = config.bold },
-     PmenuSbar = { bg = colors.bg2 },
-     PmenuThumb = { bg = colors.bg4 },
--    DiffDelete = { bg = colors.dark_red },
--    DiffAdd = { bg = colors.dark_green },
--    DiffChange = { bg = colors.dark_aqua },
--    DiffText = { bg = colors.yellow, fg = colors.bg0 },
-+    DiffDelete = { fg = colors.red },
-+    DiffAdd = { fg = colors.green },
-+    DiffChange = { fg = colors.aqua },
-+    DiffText = { fg = colors.yellow, bg = colors.bg0 },
-     SpellCap = { link = "GruvboxBlueUnderline" },
-     SpellBad = { link = "GruvboxRedUnderline" },

overlays/gruvbox-nvim-better-diff/default.nix

@@ -1,4 +0,0 @@
-self: prev:
-{
-  vimPlugins = prev.vimPlugins.extend (self.callPackage ./generated.nix { });
-}

overlays/gruvbox-nvim-better-diff/generated.nix

@@ -1,10 +0,0 @@
-{ ... }:
-
-_final: prev: {
-  gruvbox-nvim = prev.gruvbox-nvim.overrideAttrs (oa: {
-    patches = (oa.patches or [ ]) ++ [
-      # Inspired by https://github.com/ellisonleao/gruvbox.nvim/pull/291
-      ./colours.patch
-    ];
-  });
-}