releasetools: Remove deprecated BOARD_BUILD_SYSTEM_ROOT_IMAGE

This variable is always false or empty now, so clean up remaining
references to the variable.

Bug: 241346584
Test: Presubmit
Change-Id: I5dd3d356729d0bd2639633580bbb85f8b03582cf
diff --git a/tools/releasetools/validate_target_files.py b/tools/releasetools/validate_target_files.py
index 82b3107..8da4fa2 100755
--- a/tools/releasetools/validate_target_files.py
+++ b/tools/releasetools/validate_target_files.py
@@ -361,18 +361,15 @@
           "Mismatching mincrypt verity key files"
       logging.info('Verified the content of /verity_key')
 
-    # For devices with a separate ramdisk (i.e. non-system-as-root), there must
-    # be a copy in ramdisk.
-    if info_dict.get("system_root_image") != "true":
-      verity_key_ramdisk = os.path.join(
-          input_tmp, 'BOOT', 'RAMDISK', 'verity_key')
-      assert os.path.exists(
-          verity_key_ramdisk), 'Missing verity_key in ramdisk'
+    verity_key_ramdisk = os.path.join(
+        input_tmp, 'BOOT', 'RAMDISK', 'verity_key')
+    assert os.path.exists(
+        verity_key_ramdisk), 'Missing verity_key in ramdisk'
 
-      assert filecmp.cmp(
-          verity_key_mincrypt, verity_key_ramdisk, shallow=False), \
-          'Mismatching verity_key files in root and ramdisk'
-      logging.info('Verified the content of /verity_key in ramdisk')
+    assert filecmp.cmp(
+        verity_key_mincrypt, verity_key_ramdisk, shallow=False), \
+        'Mismatching verity_key files in root and ramdisk'
+    logging.info('Verified the content of /verity_key in ramdisk')
 
     # Then verify the verity signed system/vendor/product images, against the
     # verity pubkey in mincrypt format.