nvim: Only start ltex LSP if spellchecking enabled

Since ltex-lsp eats quite a lot of resources and takes a while to start
up we don't always want it enabled for every prose file. This commit
ensures that it only starts up when spellchecking is enabled for a
buffer (through the custom user command `SpellToggle`).

nvim/.config/nvim/lua/plugins/prose.lua

@@ -182,8 +182,6 @@ local prose_plugs = {
 	{
 		"barreiroleo/ltex_extra.nvim",
 		branch = "dev",
-		ft = writing_ft,
-		opts = {},
 	},
 }