Fix #142982: Grease Pencil: Assert when overriding keyframes in dopesheet

Assert when overlapping keys are removed. This is due to drawing count
mismatch, more specifically when drawing is not removed/overwritten but
early check validation caused it. Remove `validate_drawing_user_counts`
from `remove_drawings_with_no_users`. If Drawing is actually removed
(`drawings_to_remove` non empty), validation function will be called at
the end.

Pull Request: https://projects.blender.org/blender/blender/pulls/143057

source/blender/blenkernel/intern/grease_pencil.cc

@@ -3163,10 +3163,6 @@ void GreasePencil::remove_drawings_with_no_users()
   using namespace blender;
   using namespace blender::bke::greasepencil;
 
-#ifndef NDEBUG
-  this->validate_drawing_user_counts();
-#endif
-
   /* Compress the drawings array by finding unused drawings.
    * In every step two indices are found:
    *   - The next unused drawing from the start