[ntuple] Remove RFieldBase::GetNElements() #13051

Job	Run time
Windows 10 x64 Release	1h 1m 45s
Windows 10 x86 Release	58m 4s
fedora40 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	50m 12s
alma8 LLVM_ENABLE_ASSERTIONS=On	50m 31s
mac13 ARM64 LLVM_ENABLE_ASSERTIONS=On, builtin_zlib=ON	28m 2s
Upload Event Payload	3s
mac14 X64 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	53m 27s
alma9 LLVM_ENABLE_ASSERTIONS=On, CMAKE_BUILD_TYPE=Debug	1h 27m 40s
mac15 ARM64 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	27m 18s
ubuntu20 LLVM_ENABLE_ASSERTIONS=On	31m 59s
mac-beta ARM64 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	27m 59s
ubuntu22 imt=Off, LLVM_ENABLE_ASSERTIONS=On, CMAKE_BUILD_TYPE=Debug	1h 3m 54s
ubuntu2404 LLVM_ENABLE_ASSERTIONS=On, CMAKE_BUILD_TYPE=Debug	1h 8m 2s
debian125 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	55m 10s
alma9 modules_off runtime_cxxmodules=Off	42m 10s
alma9 march_native CMAKE_BUILD_TYPE=RelWithDebInfo, CMAKE_CXX_FLAGS=-march=native, CMAKE_C_FLAGS=-march=native, fortran=OFF	43m 29s
alma9 arm64 CMAKE_BUILD_TYPE=RelWithDebInfo, ccache=ON	36m 8s
alma9-clang clang LLVM_ENABLE_ASSERTIONS=On, CMAKE_C_COMPILER=clang, CMAKE_CXX_COMPILER=clang++	40m 39s
ubuntu2404-cuda gpu	1m 22s
	13h 47m 54s

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