Removed FFT code duplication

[fweik]

Removed 2nd identical implementation of 3d FFT for dipolar p3m.

[codecov]

Codecov Report

Merging #2455 into python will decrease coverage by <1%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           python   #2455    +/-   ##
=======================================
- Coverage      70%     70%    -1%     
=======================================
  Files         488     487     -1     
  Lines       30863   30639   -224     
=======================================
- Hits        21747   21563   -184     
+ Misses       9116    9076    -40

Impacted Files	Coverage Δ
src/core/electrostatics_magnetostatics/p3m.hpp	100% <ø> (ø)	⬆️
...core/electrostatics_magnetostatics/p3m-dipolar.hpp	100% <ø> (ø)	⬆️
src/core/electrostatics_magnetostatics/icc.cpp	64% <ø> (ø)	⬆️
src/core/electrostatics_magnetostatics/p3m.cpp	83% <100%> (ø)	⬆️
...core/electrostatics_magnetostatics/p3m-dipolar.cpp	79% <100%> (ø)	⬆️
src/core/electrostatics_magnetostatics/fft.cpp	88% <100%> (-1%)	⬇️

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