core: random: Factored out v_noise

[fweik]

Description of changes:

• Factored out v_noise function
• Removed unused code

[codecov]

Codecov Report

Merging #3349 into python will decrease coverage by <1%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           python   #3349   +/-   ##
======================================
- Coverage      86%     86%   -1%     
======================================
  Files         538     537    -1     
  Lines       25312   25283   -29     
======================================
- Hits        21785   21756   -29     
  Misses       3527    3527

Impacted Files	Coverage Δ
src/core/random.cpp	68% <ø> (-1%)	⬇️
src/core/dpd.cpp	92% <100%> (-1%)	⬇️
src/core/random.hpp	70% <100%> (+26%)	⬆️
src/core/thermostat.hpp	96% <100%> (-1%)	⬇️
src/core/bonded_interactions/thermalized_bond.hpp	100% <100%> (ø)	⬆️
...ore/grid_based_algorithms/lb_particle_coupling.cpp	98% <100%> (-1%)	⬇️
src/utils/include/utils/u32_to_u64.hpp

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[jngrad]

LGTM