extract_utils: fix unslot_partition() for unslotted system_ext

Prior to this change, it'd unslot "system_ext" into "system" and
overwrite system.img with it.

Change-Id: I968bf9c7f26a87ba1668d74dccc21be3a0641a8b

Author: luk1337

extract_utils/extract.py

@@ -435,7 +435,7 @@ def extract_sparse_raw_imgs(file_paths: List[str], output_dir: str):
 
 
 def unslot_partition(partition_slot: str):
-    return partition_slot.rsplit('_', 1)[0]
+    return re.sub(r'_[abc]$', '', partition_slot)
 
 
 def _extract_super_img(