Fix gmsh reader for files in MeshFormat 2

[Davknapp]

Bug report

Meshes in the written by gmesh in MeshFormat 2 are currently not readable.
Describe the bug
Reading a .msh file in MeshFormat 2 results in a
"Assertion 'class_entry->id == gtree_it'

To Reproduce
Which command did you run/which function did you call?
try to read a msh file in version 2
Expected behavior
A clear and concise description of what you expected to happen.
success
Log files

• If possible, add any output and error messages that you got
• Please upload the config.log file (when building with autotools)
• For failing tests, please upload the test-suite.log file (when building with autotools)
• When building with CMake please upload
  • CMakeCache.txt
  • CMakeFiles/CMakeError.log
  • CMakeFiles/CMakeOutput.log
  • Testing/Temporary/FailedTest.log (for failing tests)
  • Testing/Temporary/LastTestsFailed.log (for failing tests)

Environment (please complete the following information):

• OS: [e.g. Ubuntu 22.04, Windows 11]
• Compiler: [e.g., gcc, icc, mpicc]
• Compiler version: [e.g. the output of 'gcc --version']
• MPI version (if MPI is used): [The output of 'mpirun --version' or 'mpiexec --version']

Estimated priority
Which of these is most applicable (remove the others):

"Priority: high" Should be solved as soon as possible

"Priority: medium" Should be solved within half a year

"Priority: low" Should be solved eventually

Additional context
Add any other context about the problem here.

[spenke91]

In Monday's dev meeting, we agreed to remove the MeshFormat 2 support as it is causing problems without being really used. Unless you, @Davknapp , have any objection, I will close this issue and open one for removing the MeshFormat 2 support.

[Davknapp]

I have no objections

[sandro-elsweijer]

ping @spenke91

[sandro-elsweijer]

Also needed for DLR-AMR/T8code.jl#105