awscli: remove unnecessary override on python prompt_toolkit

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