lib: image_lib: drop partition flags attribute

The parted & cgpt logic have been returning different values for the
"flags" partition attribute.  The parted logic returns a string in
the format parted uses (e.g. "boot") while cgpt returns a completely
different format (e.g. "[0]"), and historically cgpt would always
set it to "".

Fortunately, no code seems to be using this field, so delete it.

BUG=b:256896261
TEST=CQ passes

Change-Id: Ic5d39ebc95beb5e59de27a767efdd895eb251a7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4163302
Reviewed-by: Sergey Frolov <sfrolov@google.com>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/test_image_unittest.py b/scripts/test_image_unittest.py
index 33d5ca7..389a316 100644
--- a/scripts/test_image_unittest.py
+++ b/scripts/test_image_unittest.py
@@ -36,9 +36,9 @@
             input="label: dos",
         )
         fake_partitions = (
-            image_lib.PartitionInfo(1, 0, 0, "fs", "STATE", "flag"),
-            image_lib.PartitionInfo(2, 0, 0, "fs", "KERN-A", "flag"),
-            image_lib.PartitionInfo(3, 0, 0, "fs", "ROOT-A", "flag"),
+            image_lib.PartitionInfo(1, 0, 0, "fs", "STATE"),
+            image_lib.PartitionInfo(2, 0, 0, "fs", "KERN-A"),
+            image_lib.PartitionInfo(3, 0, 0, "fs", "ROOT-A"),
         )
         self.PatchObject(
             image_lib,