remove tools.file_io

[jaygambetta]

What is the expected enhancement?

What is the point of this tool. As we clean up tools I want to try and make sure the tools are extensions to qiskit that don't affect the core but make it simpler to use.

[diego-plan9]

For reference, it was introduced in #116 - but I also suspect is has not really seen widespread usage, specially after the changes related to Qobj and result.

[diego-plan9]

Self-assigning, as is quite related to the results revamp at #1266 - once it is in place, saving and loading results should be just a matter of deserializing/serializing them from/to the class. Still to be decided if we deprecate or remove entirely (I'd actually lean towards just removing directly, assuming the functions are not really in use).