home-manager: remove unsupported `-2` option
home-manager: remove unsupported `-2` option

This option used to make the `home-manager` command use the `nix` tool
from Nix 2. Unfortunately the `nix` tool is a bit experimental and it
is best to await its stabilization before supporting it in Home
Manager.

rycee/home-manager,rycee/home-manager,rycee/home-manager