Cleanup: Silence unused-private-field warning

Warning message: `warning: private field 'level' is not used [-Wunused-private-field]`

Pull Request: https://projects.blender.org/blender/blender/pulls/114372

source/blender/blenloader/intern/writefile.cc

@@ -208,7 +208,6 @@ class ZstdWriteWrap : public WriteWrap {
   int next_frame = 0;
   int num_frames = 0;
 
-  int level = 0;
   ListBase frames = {};
 
   bool write_error = false;