Fix errors found by -Werror

"'const' type qualifier on return type has no effect" is one of the
errors, and an unused parameter for another.

Change-Id: Ib4bc245d992c1fc1bea0b25d5396871ca1df8411

internal/ceres/compressed_row_sparse_matrix.h

@@ -140,7 +140,7 @@ class CERES_EXPORT_INTERNAL CompressedRowSparseMatrix : public SparseMatrix {
   const int* rows() const { return &rows_[0]; }
   int* mutable_rows() { return &rows_[0]; }
 
-  const StorageType storage_type() const { return storage_type_; }
+  StorageType storage_type() const { return storage_type_; }
   void set_storage_type(const StorageType storage_type) {
     storage_type_ = storage_type;
   }

internal/ceres/evaluation_callback_test.cc

@@ -254,7 +254,7 @@ class IncrementingEvaluationCallback : public EvaluationCallback {
     counter_ += 1.0;
   }
 
-  const double counter() const { return counter_; }
+  double counter() const { return counter_; }
 
  private:
   double counter_ = -1;

internal/ceres/suitesparse.h

@@ -332,7 +332,7 @@ class SuiteSparse {
     return false;
   }
 
-  void Free(void* arg) {}
+  void Free(void* /*arg*/) {}
 };
 
 }  // namespace internal