feat(treesitter)!: remove g:ts_highlight_lua (#22257)

This variable was only meant for easy testing during the development
cycle for treesitter highlighting while Lua was the only parser useable
for daily driving. Now that we have a good vimdoc parser, this approach
simply doesn't scale and should be removed sooner rather than later.

Instead of setting this variable, people for now should add the autocommand
directly to their config:
```lua
vim.api.nvim_create_autocmd('FileType', {
  pattern = 'lua', -- or { 'lua', 'help' }
  callback = function() vim.treesitter.start() end,
})
```
(or put `vim.treesitter.start()` in an `ftplugin`).

runtime/ftplugin/lua.lua

@@ -1,3 +0,0 @@
-if vim.g.ts_highlight_lua then
-  vim.treesitter.start()
-end