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.

src/nvim/options.lua

@@ -366,7 +366,7 @@ local options = {
       desc = [=[
         When set to "dark" or "light", adjusts the default color groups for
         that background type.  The |TUI| or other UI sets this on startup
-        (triggering |OptionSet|) if it can detect the background color.
+        if it can detect the background color.
 
         This option does NOT change the background color, it tells Nvim what
         the "inherited" (terminal/GUI) background looks like.