Remove obsolete (?) matrix-with-mem methods

[fingolfin]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.01%. Comparing base (af41778) to head (388fc02).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #396      +/-   ##
==========================================
- Coverage   73.01%   73.01%   -0.01%     
==========================================
  Files          43       43              
  Lines       18378    18370       -8     
==========================================
- Hits        13419    13412       -7     
+ Misses       4959     4958       -1     

Files with missing lines	Coverage Δ
gap/projective/classicalnatural.gi	87.85% <100.00%> (+<0.01%)	⬆️

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