Remove deprecated functionality from extensions, qasm, and dagcircuit modules

[mtreinish]

Summary

This commit goes through qiskit.extensions, qiskit.qasm, qiskit.util,
and qiskit.dagcircuit modules and cleans up deprecated functionality
that has gone through a full deprecation cycle and no longer needs to
be supported.

Details and comments

[jakelishman]

We may need to leave qiskit.util another cycle, but issue a proper deprecation warning out of it. Of what I've got cloned, Aer and the old IBMQ still reference the old form, and looking at qiskit/util.py, it's not actually issuing any deprecation warnings, so people may not be aware. (See the two PRs linked above.)

For linking purposes: Ali also has #7011 to clean up qiskit.circuit.

[mtreinish]

Hmm, yeah I'll back it out the qiskit.util removal from this pr and do it later. We couldn't add deprecation warnings because the only way I could think of doing it besides add a wrapper function for each function in the module was to use a module getattr function. But that was added in python 3.7 so it wouldn't work on 3.6.

[jakelishman]

Why not just issue the deprecation warning raw in the util.py file? As long as qiskit/__init__.py doesn't import it, it won't get triggered until a user does the import, and modern Terra doesn't import it.

[jakelishman]

I mean something like having the file be:

import warnings
from qiskit.utils import *

warnings.warn("Use qiskit.utils instead", DeprecationWarning)

It isn't imported by Terra during init, so users won't see the warning until they import it themselves, and you can't use any functions through its namespace without importing it. The warning can be attached to the import, rather than the member access, I think?

[mtreinish]

I did try that when I first pushed up #5420 but back then qiskit.util was getting imported by things in the root of the package (probably from aer or ibmq which at the time got imported if available for qiskit.Aer and qiskit.IBMQ). But, yeah we can do that now as things have shifted around it should work fine. Do you want to push up a PR adding the warning?

[jakelishman]

Sure - I'll do it in a bit. I want to go through and check the other downstream qiskit packages first for uses of qiskit.util, and see if there's a sensible way we can put in the deprecation warning without heavily leaking that through to users; the functions in qiskit.utils are often the type that get actually run during module initialisation, so there's a good chance even importing Aer/whatever will show something to users (though perhaps #7204 will help with that).

[jakelishman]

These deprecations look good to me now.

[coveralls]

Pull Request Test Coverage Report for Build 1425762814

• 38 of 39 (97.44%) changed or added relevant lines in 29 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 82.496%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/tools/jupyter/version_table.py	0	1	0.0%

Totals
Change from base Build 1424775756:	0.1%
Covered Lines:	49618
Relevant Lines:	60146

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💛 - Coveralls