chore(nx): remove leftover Nx wrapper artifacts (#17916)

#17147 removed the root ./nx script, but wrapper-mode artifacts were
still present (installation in
[nx.json](https://github.com/twentyhq/twenty/blob/main/nx.json) and
tracked
[nxw.js](https://github.com/twentyhq/twenty/blob/main/.nx/nxw.js),
leaving an inconsistent setup.

This PR completes that cleanup by:

- removing installation from nx.json
- deleting tracked nxw.js
- ignoring nxw.js to prevent accidental re-introduction

Validated that Nx still works via yarn nx / npx nx.

.gitignore

@@ -10,6 +10,7 @@
 .nx/installation
 .nx/cache
 .nx/workspace-data
+.nx/nxw.js
 
 .pnp.*
 .yarn/*