Removes unused imports from code block (backport #13261)

[mergify]

Summary

Removed unused variable from code block.

Details and comments

QuantumRegister and ClassicalRegister are imported in example, but not otherwise used in the code example.

(Also resolves issue opened in Qiskit/documentation#2056)

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This is an automatic backport of pull request #13261 done by Mergify.

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[coveralls]

Pull Request Test Coverage Report for Build 11157249738

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 23 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.02%) to 89.732%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/expr.rs	1	94.02%
crates/qasm2/src/lex.rs	4	92.23%
crates/qasm2/src/parse.rs	18	96.23%

Totals
Change from base Build 11148098131:	-0.02%
Covered Lines:	66874
Relevant Lines:	74526

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💛 - Coveralls