This removes the file nix-mode.el from Nix. The file is now available within the
repository https://github.com/NixOS/nix-mode.

Fixes #662
Fixes #1040
Fixes #1054
Fixes #1055
Closes #1119
Fixes #1419

NOTE: all of the above should be fixed within NixOS/nix-mode. If one of those
hasn’t please reopen within NixOS/nix-mode and not within NixOS/nix.