Remove optional sympy imports

[TsafrirA]

Summary

#10902 promoted symengine to a hard requirement with the aim of supporting only it, and not sympy. However, few optional sympy imports remained in the tests. This PR removes these optional imports.

[qiskit-bot]

One or more of the the following people are requested to review this:

• @Qiskit/terra-core
• @mtreinish
• @nkanazawa1989

[ElePT]

LGTM

Just realized that the formatting CI run is failing, I'll re-approve once you run tox -eblack

[coveralls]

Pull Request Test Coverage Report for Build 7697207518

Warning: This coverage report may be inaccurate.

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A quick fix for this PR: rebase it; your next report should be accurate.

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.004%) to 89.555%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	2	91.94%

Totals
Change from base Build 7695410601:	0.004%
Covered Lines:	59555
Relevant Lines:	66501

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💛 - Coveralls