Other: Move mains out off T8_EXTERN_C region

[benegee]

Describe your changes here:

Noticed today while compiling t8code main using gcc (GCC) 15.2.1 20260103

[ 98%] Building CXX object example/CMakeFiles/t8_example_spheres.dir/remove/t8_example_spheres.cxx.o
/home/bene/adaptex/t8code/example/remove/t8_example_spheres.cxx:129:1: warning: cannot declare ‘::main’ with a linkage specification [-Wpedantic]
  129 | main (int argc, char **argv)
      | ^~~~

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.95%. Comparing base (9f94c4e) to head (9a3389f).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2131   +/-   ##
=======================================
  Coverage   77.95%   77.95%           
=======================================
  Files         113      113           
  Lines       19089    19089           
=======================================
  Hits        14881    14881           
  Misses       4208     4208           

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[lenaploetzke]

I am not sure if we even need T8_EXTERN_C_BEGIN (); and END() in the examples. Should this code be callable from C?

[benegee]

To be honest, I don't know. @sandro-elsweijer ?

[sandro-elsweijer]

We sometimes build tutorials on top of each other, but with examples we don't do that. So here we can remove them pretty safely :)

[benegee]

Removed!

[lenaploetzke]

Thank you! :)