fix(startup): emitting useless OptionSet #39830

Problem:
During startup, we manually trigger a useless and misleading `OptionSet`
event, which doesn't set `v:option_*` values (this is a limitation of
`nvim_exec_autocmds`).
ad4bc2d90c/runtime/lua/vim/_core/defaults.lua (L939).

Solution:
The `nvim_exec_autocmds('OptionSet',…)` call does not serve any purpose
since 5cbb9d613b, so just drop it.

runtime/lua/vim/_core/defaults.lua

@@ -946,20 +946,6 @@ do
             local luminance = (0.299 * rr) + (0.587 * gg) + (0.114 * bb)
             local bg = luminance < 0.5 and 'dark' or 'light'
             vim.api.nvim_set_option_value('background', bg, {})
-
-            -- Ensure OptionSet still triggers when we set the background during startup
-            if vim.v.vim_did_enter == 0 then
-              vim.api.nvim_create_autocmd('VimEnter', {
-                group = group,
-                once = true,
-                nested = true,
-                callback = function()
-                  vim.api.nvim_exec_autocmds('OptionSet', {
-                    pattern = 'background',
-                  })
-                end,
-              })
-            end
           end
         end
       end)