Rename get_matrix/get_eigvals to matrix/eigvals and remove properties with same name

[mariaschuld]

Context:

To make the transition from op.matrix to op.matrix() smoother, we used the temporary name get_matrix for the latter for 2 release cycles. The same is true for op.eigvals. This PR removes the property and introduces the final name.

Description of the Change:

• Removed the two properties
• Refactored the two functions
• Removed deprecation tests

As a tangential extra, I also removed the warning that decomposition now is a method, which has been in the code base for a lot of releases now.

Benefits:

Finalises implementations of the operator refactor. Using methods gives us a lot more flexibility (i.e. to have matrix() take wire_order as an argument, and makes explicit that heavy computations may be executed.

Possible Drawbacks:

User code relying on the properties breaks now.

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

Merging #2498 (54ff26d) into master (bd552ac) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2498      +/-   ##
==========================================
- Coverage   99.48%   99.47%   -0.01%     
==========================================
  Files         243      243              
  Lines       19432    19413      -19     
==========================================
- Hits        19331    19312      -19     
  Misses        101      101              

Impacted Files	Coverage Δ
pennylane/gradients/parameter_shift.py	100.00% <ø> (ø)
pennylane/templates/subroutines/qpe.py	100.00% <ø> (ø)
pennylane/transforms/commutation_dag.py	99.69% <ø> (ø)
pennylane/transforms/control.py	100.00% <ø> (ø)
pennylane/transforms/op_transforms.py	100.00% <ø> (ø)
pennylane/_qubit_device.py	98.73% <100.00%> (ø)
pennylane/devices/default_mixed.py	100.00% <100.00%> (ø)
pennylane/devices/default_qubit.py	100.00% <100.00%> (ø)
pennylane/devices/default_qubit_torch.py	92.07% <100.00%> (ø)
pennylane/grouping/utils.py	100.00% <100.00%> (ø)
... and 10 more

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[mariaschuld]

I am not sure why these sections show as "added", when only the >>> op.eigvals() line was changed...

[mariaschuld]

I'm not quite sure what codecov is complaining about, since I only renamed and deleted code...

[josh146]

A very nice, straightforward PR to review! Only comment is regarding the decomposition, which should also be mentioned in the changelog

[josh146]

👍

[mariaschuld]

Codecov seems to complain about having less code, not more uncovered code...will overwrite