fix: remove incorrect assertion

Note that `get_cases_on_minors_range` is now parametric on `m_before_erasure`:
`get_cases_on_minors_range(env(), const_name(fn), m_before_erasure)`

src/library/compiler/csimp.cpp

@@ -330,7 +330,6 @@ class csimp_fn {
     };
 
     void collect_cases_info(expr e, cases_info_result & result) {
-        lean_assert(m_before_erasure);
         while (true) {
             if (is_lambda(e))
                 e = binding_body(e);