Remove deprecated methods from QuantumCircuit

[jakelishman]

Summary

This removes the old c?u[1-3] methods from QuantumCircuit, along with the combine and extend methods. The numbered-argument UGate alternatives had been deprecated since Terra 0.16, and the simpler forms of compose since 0.17.

The U1Gate class, and so on, are left in so the transpiler and equivalence rules can still support legacy backends with these as basis gates, but their privileged place as direct methods on QuantumCircuit is removed.

Details and comments

I also cut out the test warning suppressions that were related to these methods.

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[Cryoris]

LGTM, always nice to clean up the code 😉 There's a lint error due to an unused import of warnings in the test_extensions_standard but otherwise this looks good!

[jakelishman]

Updated, thanks!

[coveralls]

Pull Request Test Coverage Report for Build 3380611518

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 84.51%

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Files with Coverage Reduction	New Missed Lines	%
qiskit/pulse/library/waveform.py	3	91.49%

Totals
Change from base Build 3380350279:	0.1%
Covered Lines:	62359
Relevant Lines:	73789

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💛 - Coveralls

[jakelishman]

I've removed the now-unused private circuit method _check_compatible_regs to make the coverage check happier. That function wasn't sufficient as a check anyway (not to mention hideously inefficient...), and compose does it a bit better.

[jakelishman]

@Mergifyio requeue

[mergify]

requeue

✅ The queue state of this pull request has been cleaned. It can be re-embarked automatically