Fix a memory leak bug in function "lv_objmask_remove_mask" .
Fix a memory leak bug in function "lv_objmask_remove_mask" .

There is a little memory leak When you call function "lv_objmask_remove_mask" to remove a specified mask.
It's need free the memory of list's node.
littlevgl/lvgl,littlevgl/lvgl,littlevgl/lvgl,littlevgl/lvgl