Eigen/Sparse: fix warnings -Wunused-but-set-variable

What does this implement/fix?

This merge-request fixes the following warnings, reported by clang 16.0.0git (compiled from the main branch of LLVM/clang) in Eigen/Sparse:

/usr/include/eigen3/Eigen/src/SparseLU/SparseLU_heap_relax_snode.h:78:9: warning: variable 'nsuper_et_post' set but not used [-Wunused-but-set-variable]
  Index nsuper_et_post = 0; // Number of relaxed snodes in postordered etree
        ^
/usr/include/eigen3/Eigen/src/SparseLU/SparseLU_heap_relax_snode.h:79:9: warning: variable 'nsuper_et' set but not used [-Wunused-but-set-variable]
  Index nsuper_et = 0; // Number of relaxed snodes in the original etree
        ^

/usr/include/eigen3/Eigen/src/SparseCore/TriangularSolver.h:273:13: warning: variable 'count' set but not used [-Wunused-but-set-variable]
      Index count = 0;
            ^

Additional information: Context

Those warnings can be seen in context in the testsuite page of CGAL (The Computational Geometry Algorithms Library) and in particular here: https://cgal.geometryfactory.com/CGAL/testsuite/CGAL-5.6-I-86/Heat_method_3/TestReport_gimeno_Debian-testing-clang-main.gz.

Additional information: Testsuite passed locally

I have compiled and run tests successfully on my machine (x86_64, with compiler clang version 14.0.5 (Fedora 14.0.5-1.fc36)).

100% tests passed, 0 tests failed out of 1121

Label Time Summary:
Official       = 8562.54 sec*proc (778 tests)
Unsupported    = 1867.33 sec*proc (233 tests)
smoketest      = 176.24 sec*proc (109 tests)

Total Test time (real) = 1041.58 sec