[Hotfix] Removed unused variables

[bodhinandach]

Describe the PR Removing some unused variables initiated.

Related Issues/PRs Fix partially issue #658, however, instead of using fbinfer (https://fbinfer.com) I used -DCMAKE_CXX_FLAGS="Wunused-variable" flags to note. The list noted by fbinfer (https://fbinfer.com) is somehow not accurate.

[codecov[bot]]

Codecov Report

Merging #659 into develop will decrease coverage by 0.01%. The diff coverage is 60.00%.

@@             Coverage Diff             @@
##           develop     #659      +/-   ##
===========================================
- Coverage    96.55%   96.55%   -0.01%     
===========================================
  Files          122      122              
  Lines        25395    25386       -9     
===========================================
- Hits         24520    24509      -11     
- Misses         875      877       +2     

Impacted Files	Coverage Δ
include/elements/2d/triangle_quadrature.tcc	100.00% <ø> (ø)
include/materials/modified_cam_clay.tcc	89.10% <ø> (-0.04%)	:arrow_down:
include/materials/mohr_coulomb.tcc	99.29% <ø> (-<0.01%)	:arrow_down:
include/mpi_datatypes.h	100.00% <ø> (ø)
include/node.tcc	95.79% <ø> (-0.01%)	:arrow_down:
include/solvers/mpm_base.tcc	69.93% <0.00%> (-0.36%)	:arrow_down:
tests/io/write_mesh_particles.cc	88.10% <ø> (-0.06%)	:arrow_down:
tests/io/write_mesh_particles_unitcell.cc	88.10% <ø> (-0.06%)	:arrow_down:
include/mesh.tcc	84.10% <100.00%> (-0.04%)	:arrow_down:

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[bodhinandach]

@kks32 One question: How do you want to deal with these lines? Do you want to still check assert? bad_phase is unused at the moment. https://github.com/cb-geo/mpm/blob/8472fb7f3c0e30fbec71507cca1578469d510769/tests/node_test.cc#L191-L198