[PWGCF] twoParticleCorrelationsMultSpher: Address review comments

- Remove dead code (#if 0 blocks, unused histograms/axes/parameters)
- Use RecoDecay::constrainAngle instead of custom computeDeltaPhi
- Use O2DatabasePDG service instead of TDatabasePDG
- Replace hard-coded bin logic with edge arrays and o2::analysis::findBin
- Generate multiplicity labels from the bin edges
- Fix include grouping and remove comments between includes

Author: matarzil