VCS: Remove project_path VCS property, let plugins decide

[twaritwaikar]

Screenshots:

On a completely new repo (no .git directory in current or parent paths)

On a seasoned repo (parent path has a .git directory)

This PR was made in parallel with - twaritwaikar/godot-git-plugin#1

The project path is no longer required because if the plugin can self-identify the project directory, then there is no need for the user to customize this behaviour. Using a bad project path will result in frustrating errors since we expect the plugin to work the same way as the git CLI.

[twaritwaikar]

Can be cherrypicked to 3.x
I will backport this to 3.x

[akien-mga]

Thanks!