[PWGLF] Quick fix for TH3 merging issue

[vbarbaso]

• The labelled Z axis of hNPhiPtCent was causing incorrect behaviour during O2/Hyperloop merging. The merged histogram could acquire an extended Z axis and shift the bin contents into the overflow region.
• Removing the bin labels keeps the axis as a fixed numerical axis and ensures that the histogram is merged correctly while leaving the binning and filling unchanged.

[github-actions]

O2 linter results: ❌ 0 errors, ⚠️ 0 warnings, 🔕 0 disabled

[alibuild]

Error while checking build/O2Physics/code-check for daf2782 at 2026-09-14 09:48:

## sw/BUILD/O2Physics-code-check-latest/log
--
========== List of issues found ==========
++ echo 'Found 1 errors and 0 warnings.'
Found 1 errors and 0 warnings.
++ [[ 1 -gt 0 ]]
++ cat /sw/BUILD/9cd146f02d0dfbf7c6e17e5cfba398a1d72d805c/O2Physics-code-check/errors.txt
PWGLF/Tasks/Resonances/phianalysisTHnSparse.cxx:52:1: error: included header TH3.h is not used directly [misc-include-cleaner,-warnings-as-errors]
++ [[ 0 -gt 0 ]]
++ [[ 1 -gt 0 ]]
++ exit 1
--

Full log here.