Delete the unused variable i_cell

GCC 16.1.0 in MinGW warns that the variable 'i_cell' set but not used.

tests/gtest/avifincrtest_helpers.cc

diff --git a/tests/gtest/avifincrtest_helpers.cc b/tests/gtest/avifincrtest_helpers.cc
index 70e485b..efa55d4 100644
--- a/tests/gtest/avifincrtest_helpers.cc
+++ b/tests/gtest/avifincrtest_helpers.cc
@@ -211,8 +211,8 @@
 
   std::vector<ImagePtr> cell_images;
   cell_images.reserve(grid_cols * grid_rows);
-  for (uint32_t row = 0, i_cell = 0; row < grid_rows; ++row) {
-    for (uint32_t col = 0; col < grid_cols; ++col, ++i_cell) {
+  for (uint32_t row = 0; row < grid_rows; ++row) {
+    for (uint32_t col = 0; col < grid_cols; ++col) {
       avifCropRect cell;
       cell.x = col * *cell_width;
       cell.y = row * *cell_height;