Remove unneeded casts

[mrossinek]

Summary

This removes some unneeded casts from the operator classes.

Details and comments

Complexity should not pose a real problem any longer because we do not do any explicit down- or up-conversions in the stack any longer. This can be seen since Tensor and PolynomialTensor explicitly allow complex values. I do not recall why I added these casts in the first place, but I think they can simply be removed.

[mrossinek]

If approved, should we backport this to stable, too?

EDIT: no, there is no need to do this

[coveralls]

Pull Request Test Coverage Report for Build 4812686508

• 4 of 6 (66.67%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.009%) to 85.86%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit_nature/second_q/operators/spin_op.py	1	2	50.0%
qiskit_nature/second_q/operators/vibrational_op.py	1	2	50.0%

Totals
Change from base Build 4809087501:	0.009%
Covered Lines:	18587
Relevant Lines:	21648

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💛 - Coveralls