[doc] Remove obsolete nonuniform tags docu.

...ry_condition/NonuniformDirichlet/c_NonuniformDirichlet.md

...ns/boundary_condition/NonuniformDirichlet/t_field_name.md

...undary_condition/NonuniformNeumann/c_NonuniformNeumann.md

...ions/boundary_condition/NonuniformNeumann/t_field_name.md

...eDependentNeumann/c_NonuniformVariableDependentNeumann.md

...leDependentNeumann/t_coefficient_current_variable_name.md

...bleDependentNeumann/t_coefficient_mixed_variables_name.md

...ableDependentNeumann/t_coefficient_other_variable_name.md

...ary_condition/VariableDependentNeumann/t_constant_name.md