[tmva] Remove unused LinkDef5.h

[kutsibalci]

What

Removes tmva/tmva/inc/LinkDef5.h, which has not been part of any dictionary since 2015.

Why

The split of TMVA into libTMVA and libTMVAGui (5f684bf, 2015-04-28) dropped the
#include "LinkDef5.h" line from tmva/tmva/inc/LinkDef.h but left the file itself in place.
It has been unreachable ever since:

• tmva/tmva/inc/LinkDef.h includes only LinkDef1.h through LinkDef4.h.
• No CMakeLists.txt or .cmake file in the repository refers to LinkDef5.h, and the TMVA
  package relies on the default LINKDEF value of LinkDef.h (cmake/modules/RootMacros.cmake:1364).
• Every symbol it declares is already declared in tmva/tmvagui/inc/LinkDef.h, which belongs to
  the module that actually defines those classes — TMVA::MethodInfo and TMVA::StatDialogMVAEffs
  in tmva/tmvagui/inc/TMVA/mvaeffs.h, TMVA::StatDialogBDT in tmva/tmvagui/inc/TMVA/BDT.h.
  Comparing the two files, LinkDef5.h contains no symbol that tmvagui/inc/LinkDef.h does not
  already contain, so nothing is lost by removing it.

Two later commits edited the file without noticing it was dead — eec693f (2020, dropping a
tmvaglob.h pragma) and e5f5cad (2024, the __CINT__ → __CLING__ sweep). That is the
maintenance cost this removal avoids.

Verification

Configured a build with the file removed (Ninja, -DCMAKE_BUILD_TYPE=Release -Dtesting=ON -Droottest=OFF, GCC 13.3 on Ubuntu 24.04). Configuration completes with no errors, and the
generated build.ninja contains:

reference	occurrences
LinkDef5	0
tmva/tmva/inc/LinkDef.h	2
tmva/tmvagui/inc/LinkDef.h	2

All three TMVA dictionaries are still generated: G__TMVA.cxx, G__TMVAGui.cxx, G__TMVAUtils.cxx.

AI-assisted coding disclosure

This contribution was AI-assisted (Claude Code). The tool was used to scan LinkDef files across
the repository for entries whose classes are not declared in the module that owns them, and to
draft this description.

The finding was verified before opening the PR rather than taken on trust: the file was confirmed
unreferenced repository-wide, the 2015 commit that removed the include was traced, the symbol sets
of the two LinkDef files were compared, and the configuration check above was run against the
branch with the file already deleted. Three other candidates the same scan produced
(ROOT::Internal::RDF::CountHelper, MeanHelper, BufferedFillHelper) were false positives
caused by the R__CLING_PTRCHECK(off) attribute and were discarded rather than reported.

I have reviewed and understood the change and take responsibility for it.

[guitargeek]

LGTM! Indeed, LinkDef5.h is not referred to anywhere in the source tree.