Remove .node-version file from repo root and template

Summary:
IMO, there's no value in having `.node-version` files either in the repository or the template.

I don't believe it's standard practice, the support from tooling is patchy (at least `nvm` [ignores it](nvm-sh/nvm#794), and Node is often used without a version manager), and it's unnecessarily restrictive when it is actually enforced. In every other case, it's potentially misleading noise.

Libraries should use `package.json#engines` to specify their Node JS compatibility ranges, as indeed `react-native` [already does](https://github.com/facebook/react-native/blob/1629b9f0a11f42ff12c6dd06a20afb80067e4958/package.json#L10). React Native users may wish to add one to their own projects according to their own setup or team, but we shouldn't confuse the issue by including an arbitrary one in the template.

Changelog:
[General][Removed] Remove `.node_version` from app template.

Differential Revision: D43421085

fbshipit-source-id: 37558ef3aee6479b40746cccf83beb1577473786