Docgen(fix): Remove formatCode function

[aspeddro]

For some reason formatCode don`t print signature for some externals, see Belt.Int

Shared.{type,decl}ToString already does the formatting

Example:

external \"-": (int, int) => int = "%subint"

Before:

{
  "name": "Lib",
  "docstrings": [],
  "items": [
  {
    "id": "Lib.-",
    "kind": "value",
    "name": "-",
    "signature": "let _: %rescript.typehole",
    "docstrings": []
  }]
}

After:

{
  "name": "Lib",
  "docstrings": [],
  "items": [
  {
    "id": "Lib.-",
    "kind": "value",
    "name": "-",
    "signature": "let -: (. int, int) => int",
    "docstrings": []
  }]
}