Remove cylindrical_average.

[pkreissl]

Fixes #3450

[codecov]

Codecov Report

Merging #3470 into python will increase coverage by <1%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           python   #3470    +/-   ##
=======================================
+ Coverage      86%     86%   +<1%     
=======================================
  Files         537     537            
  Lines       24721   24659    -62     
=======================================
+ Hits        21333   21335     +2     
+ Misses       3388    3324    -64

Impacted Files	Coverage Δ
src/core/statistics.cpp	75% <ø> (+11%)	⬆️
src/core/statistics.hpp	87% <ø> (ø)	⬆️
src/core/electrostatics_magnetostatics/p3m.cpp	85% <0%> (ø)	⬆️

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