Remove Sympy simulators for moving to new repository

[yaelbh]

Addressing Issue #425, I created a new repository for Sympy simulators. In that repository, I opened a pull request, parallel to this one:
https://github.com/QISKit/qiskit-core-backend-module-sympy/pull/1

Now back to the qiskit-sdk-py repository, this pull request removes Sympy simulators, while making sure that all thet remains in qiskit-sdk-py works properly.

Issue #425 contains more detailed information about what's planned both here and in the new repository.

Description

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[diego-plan9]

Thanks for the thorough removal of the sympy simulators, @yaelbh ! I am merging this PR already after discussion on yesterday's meeting, looking forward to the simulators finding a new home in the new repo soon 🎉