Do not read the block device when generating the care map

This remove the fstab dependency when building the OTA package for
marlin/sailfish.

Bug: 35811655
Test: OTA package builds successfully for sailfish.
Change-Id: If223d11dddca396c47262042c576f9e7d0cb5b33
(cherry picked from commit 7d051adc3be56594ee755bf4ed06b6597fd62268)
diff --git a/tools/releasetools/add_img_to_target_files.py b/tools/releasetools/add_img_to_target_files.py
index 28fd474..9403a77 100755
--- a/tools/releasetools/add_img_to_target_files.py
+++ b/tools/releasetools/add_img_to_target_files.py
@@ -77,11 +77,10 @@
   """Generate care_map of system (or vendor) partition"""
 
   assert which in ("system", "vendor")
-  _, blk_device = common.GetTypeAndDevice("/" + which, OPTIONS.info_dict)
 
   simg = sparse_img.SparseImage(imgname)
   care_map_list = []
-  care_map_list.append(blk_device)
+  care_map_list.append(which)
 
   care_map_ranges = simg.care_map
   key = which + "_adjusted_partition_size"