Fix memory leak in merge macro

[cmargalejo]

• Add missing delete mergeRun after CloseFile() in REST_Geant4_MergeRestG4Files macro
• Without this, each merge operation leaks the entire TRestRun object
• Together with the TRestRun::CloseFile fix (Fix memory leak in TRestRun::CloseFile framework#562), this resolves the memory growth when merging a large number of files

Relates to rest-for-physics/framework#519

@rest-for-physics/geant4lib

[juanangp]

Looking at the merge conflit, it seems that you are working on a old version of this library.
I think that current version in which mergeRun is not a pointer shouldn't cause any memory leak, so I guess you can close this PR.

[cmargalejo]

You're right, thanks for pointing it out! I was working on an older version of the submodule where mergeRun was still a pointier. This submodule structure is not very straightforward I think... I close this.