Removing max_axial_mesh_size parameter from ABTR workflow

[shikhar413]

Reason

Doing some more cleanup of Griffin, where I am removing the parameter Mesh/EquivalentCoreMeshGenerator/max_axial_mesh_size, whic can be inferred from another part of the input file. The removal of this parameter is causing a VTB test failure, and the VTB models need to be updated. For more info see https://github.inl.gov/ncrc/griffin/pull/2118 and https://civet.inl.gov/job/2470692/

Design

Removing parameter from input file and updating documentation to explain how the axial specifications are defined for the homogenized geometry

Impact

Removal of parameter, no change to actual model itself