nvim: Remove manual nushell tresitter installation

It is no longer necessary to manually install the nushell tresitter as a
lazy plugin spec, according to:
https://github.com/nushell/tree-sitter-nu/blob/main/installation/neovim.md

Instead we just enable the treesitter 'nu' language in the
`languages.lua` file as usual and it works out of the box.

nvim/.config/nvim/lua/modules/treesitter.lua

@@ -20,7 +20,6 @@ return {
 					require("rainbow-delimiters.setup").setup({})
 				end,
 			},
-			{ "nushell/tree-sitter-nu", version = false },
 		},
 		config = function()
 			local enabled_parsers = {}