[PWGCF] Deleted the Diff_pT_fluct_PID.cxx due to the request from @vkucera

[Annabinoy]

Removing analysis files that are no longer in use.
This cleanup was discussed with Vit Kucera to prevent wasting compilation resources on obsolete code.

[alibuild]

Error while checking build/O2Physics/o2 for 9ff45ed at 2026-02-03 21:21:

## sw/BUILD/O2Physics-latest/log
CMake Error at cmake/O2PhysicsAddExecutable.cmake:113 (add_executable):
CMake Error at cmake/O2PhysicsAddExecutable.cmake:113 (add_executable):

Full log here.

[alibuild]

Error while checking build/O2Physics/o2 for b8f5230 at 2026-02-05 08:56:

## sw/BUILD/O2Physics-latest/log
CMake Error at cmake/O2PhysicsAddExecutable.cmake:113 (add_executable):
CMake Error at cmake/O2PhysicsAddExecutable.cmake:113 (add_executable):

Full log here.