Remove the duplicated docs for a BackendV1 classmethod (backport #12443)

[mergify]

The BackendV1 page has a duplicated entry for the _default_options() method as we can see in this screenshot highlight it with two red arrows.

This PR removes the duplicated autoclass directive for _default_options() and the line of documentation explaining that subclasses need to implement it, as it is also explained in line 66.

qiskit/qiskit/providers/backend.py

Lines 66 to 69 in a83c99b

	Subclasses of this should override the public method :meth:`run` and the internal
	:meth:`_default_options`:
	.. automethod:: _default_options

With this removal, the docs will match the documentation of BackendV2 where we use the same explanation once:

qiskit/qiskit/providers/backend.py

Lines 316 to 319 in a83c99b

	Subclasses of this should override the public method :meth:`run` and the internal
	:meth:`_default_options`:
	.. automethod:: _default_options

The change will need backport to stable/1.1, stable/1.0, and stable/0.46

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This is an automatic backport of pull request #12443 done by Mergify.

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[coveralls]

Pull Request Test Coverage Report for Build 9160468078

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 14 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.02%) to 89.605%

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Files with Coverage Reduction	New Missed Lines	%
qiskit/transpiler/passes/synthesis/unitary_synthesis.py	2	88.2%
crates/qasm2/src/lex.rs	6	91.86%
crates/qasm2/src/parse.rs	6	97.15%

Totals
Change from base Build 9133287908:	-0.02%
Covered Lines:	62247
Relevant Lines:	69468

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💛 - Coveralls