Fix calling deprecated submit()

[woodsp-ibm]

Summary

Resolves #126

Details and comments

I chose to do this in a way that's compatible with the currently defined min Qiskit version and avoids deprecation warnings for future versions where in PrimitiveJob its now _submit()

I altered the text about an empty class its no longer true when looking at the source, and in the docs it not appear that way at all anyway since it showed all the inherited methods. I also added a note about the why submit() exists in it now and a TODO as a reminder to potentially revisit this in the future.

[coveralls]

Pull Request Test Coverage Report for Build 7582831436

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 90.583%

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Totals
Change from base Build 7582828207:	0.005%
Covered Lines:	6474
Relevant Lines:	7147

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💛 - Coveralls