Remove unnecessary tearDown code

[jakelishman]

The _backends instance value on BasicProvider() is an instance value, so re-calculating it on each test return no longer has any effect. The corresponding value used to be a class attribute on BasicAer, but this was changed in #11422.

Summary

Details and comments

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core

[coveralls]

Pull Request Test Coverage Report for Build 19833134767

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.01%) to 88.404%

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Files with Coverage Reduction	New Missed Lines	%
crates/circuit/src/parameter/parameter_expression.rs	1	82.3%
crates/qasm2/src/lex.rs	3	92.54%

Totals
Change from base Build 19827713394:	0.01%
Covered Lines:	95754
Relevant Lines:	108314

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💛 - Coveralls

[mtreinish]

I feel like there was something with the fake providers in the need for this historically too (like the provider list was being modified at runtime by the fake provider listing somewhere. But regardless it seems safe to remove this now.

[jakelishman]

Yeah, when this was the BasicAer singleton provider I think it may have had some effect, but now the provider instantiates new simulators per instance anyway.