Remove deprecated a2idx and classof

[sylee957]

References to other Issues or PRs

Fixes #15109

Brief description of what is fixed or changed

Other comments

Release Notes

• matrices
  • BREAKING: Removed deprecated functions a2idx and classof from sympy.matrices. This had been deprecated and giving out DeprecationWarning since SymPy 1.3.

[sympy-bot]

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• matrices
  • BREAKING: Removed deprecated functions a2idx and classof from sympy.matrices. This had been deprecated and giving out DeprecationWarning since SymPy 1.3. (#24513 by @sylee957)

This will be added to https://github.com/sympy/sympy/wiki/Release-Notes-for-1.12.

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#### References to other Issues or PRs
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Fixes #15109

#### Brief description of what is fixed or changed


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#### Release Notes

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  * Added a new solver for logarithmic equations.

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<!-- BEGIN RELEASE NOTES -->
- matrices
  - BREAKING: Removed deprecated functions `a2idx` and `classof` from `sympy.matrices`. This had been deprecated and giving out `DeprecationWarning` since SymPy 1.3.
<!-- END RELEASE NOTES -->

Update

The release notes on the wiki have been updated.

[github-actions]

Benchmark results from GitHub Actions

Lower numbers are good, higher numbers are bad. A ratio less than 1
means a speed up and greater than 1 means a slowdown. Green lines
beginning with + are slowdowns (the PR is slower then master or
master is slower than the previous release). Red lines beginning
with - are speedups.

Significantly changed benchmark results (PR vs master)

Significantly changed benchmark results (master vs previous release)

       before           after         ratio
     [41d90958]       [f3b2342e]
     <sympy-1.11.1^0>                 
+     1.17±0.04ms       1.93±0.2ms     1.65  solve.TimeRationalSystemSymbol.time_solve(1)
-      3.46±0.3ms      1.54±0.06ms     0.45  solve.TimeSparseSystem.time_linear_eq_to_matrix(20)
-      7.38±0.3ms      2.38±0.09ms     0.32  solve.TimeSparseSystem.time_linear_eq_to_matrix(30)

Full benchmark results can be found as artifacts in GitHub Actions
(click on checks at the top of the PR).