fix: remove unnecessary "bin/start-stdio" script

I think I figured out how it's supposed to work without adding
additional scripts or worrying about passed arguments.

The mason-registry submission is here: mason-org/mason-registry#8368

Once refactorex is installed with mason, it can be configured using the
following code:

```lua
require("lspconfig.configs").refactorex = {
  default_config = {
    cmd = { vim.fn.stdpath("data") .. "/mason/bin/refactorex", "--stdio" },
    filetypes = { "elixir" },
    root_dir = function(fname)
      return require("lspconfig").util.root_pattern("mix.exs")(fname)
    end,
    settings = {},
    capabilities = {
      textDocumentSync = {
        openClose = true,
        change = 1,
        save = { includeText = true },
      },
      codeActionProvider = {
        resolveProvider = true,
      },
      renameProvider = {
        prepareProvider = true,
      },
    },
  },
}

require("lspconfig").refactorex.setup({})
```

As you can see, the `cmd` has the `--stdio` argument. Mason's job is
mainly just to install the language server, not to actually say what
will be done with it.

For maximum ease, once it is accepted in Mason, we will need to also
submit it to https://github.com/williamboman/mason-lspconfig.nvim,
similar to how nextls has done it here:
https://github.com/williamboman/mason-lspconfig.nvim/blob/main/lua/mason-lspconfig/server_configurations/nextls/init.lua

```lua
return function()
  return {
    cmd = { "nextls", "--stdio" },
  }
end
```

So, mason is just to install the language server, mason-lspconfig is
what provides the easy-to-use lsp configuration. Until it's accepted in
`mason-lspconfig`, your friends can use the snippet above to get it
working (once it's accepted to mason itself).

Until it's accepted in mason, they can just use this plugin:
https://github.com/synic/refactorex.nvim - it just downloads the
refactorex source code to a local directory and runs it from there.
Seems to work just fine.

Author: synic