Improvement: Remove support of gmsh mesh format 2

[spenke91]

Closes #2071

Describe your changes here:

This PR removes all functionality related to gmsh MeshFormat 2, which we decided no longer to support in t8code.

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• The PR is small enough to be reviewed easily. If not, consider splitting up the changes in multiple PRs.
• The title starts with one of the following prefixes: Documentation:, Bugfix:, Feature:, Improvement: or Other:.
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All these boxes must be checked by the REVIEWERS before merging the pull request:

As a reviewer please read through all the code lines and make sure that the code is fully understood, bug free, well-documented and well-structured.

General

• The reviewer executed the new code features at least once and checked the results manually.
• The code follows the t8code coding guidelines.
• New source/header files are properly added to the CMake files.
• The code is well documented. In particular, all function declarations, structs/classes and their members have a proper doxygen documentation. Make sure to add a file documentation for each file!
• All new algorithms and data structures are sufficiently optimal in terms of memory and runtime (If this should be merged, but there is still potential for optimization, create a new issue).

Tests

• The code is covered in an existing or new test case using Google Test.
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• Valgrind doesn't find any bugs in the new code. This script can be used to check for errors; see also this wiki article.

If the Pull request introduces code that is not covered by the github action (for example coupling with a new library):

• Should this use case be added to the github action?
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Scripts and Wiki

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License

• The author added a BSD statement to doc/ (or already has one).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.61%. Comparing base (240b652) to head (e0ad66a).
⚠️ Report is 162 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2072      +/-   ##
==========================================
+ Coverage   77.46%   77.61%   +0.14%     
==========================================
  Files         112      112              
  Lines       19084    18934     -150     
==========================================
- Hits        14784    14696      -88     
+ Misses       4300     4238      -62     

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