awscli: remove unnecessary override on python prompt_toolkit

[bhipple]

This override to the old 1.x version of prompt_toolkit appears to be
unnecessary; removing it does not change the hash of awscli.

In a follow-up, we could likely remove the RSA override as well, if we're OK
with patching out the setup.cfg requirements. This dropped support for some
old modules, but appears to not break API compatibility otherwise:
https://github.com/sybrenstuvel/python-rsa/blob/master/CHANGELOG.md#version-40---released-2018-09-16

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @muflax

[bhipple]

Note that this does not change the hash of the package:

λ brh nixpkgs →  git checkout master
λ brh nixpkgs →  nix-build -A awscli
/nix/store/zzvsdn9irj0g011klja1hdjnibk4278w-awscli-1.16.266

λ brh nixpkgs →  git checkout fix/awscli-prompt-toolkit
λ brh nixpkgs →  nix-build -A awscli
/nix/store/zzvsdn9irj0g011klja1hdjnibk4278w-awscli-1.16.266

so it should just be a no-op cleanup of an unnecessary override.

CC @jonringer @FRidh