Revert #1440 i.e. remove PauliSumOp

[woodsp-ibm]

A PauliSumOp was introduced by #1440 however with the move of opflow to qiskit-terra the work around PauliSumOp functionality was completed there. The function added here has not been released, and the code was moved and is now updated, maintained in terra repo. It is not explicitly used in Aqua outside of opflow other than unit tests with the exception of being added into the QP convertor from_ising method. As this was the first version, and has been improved/fixed now in terra where it was moved, and has never been released in Aqua, I think its better not to introduce this at the time we are deprecating the repo so am proposing that this PR be reverted to remove PauliSumOp.

Any thoughts to the contrary @ikkoham @t-imamichi @Cryoris @stefan-woerner ? I will note that the PR would need to be reverted manually as files have changed since it was merged such that it cannot automatically create a PR that would revert the changes back again.