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/sign_target_files_apks.py b/tools/releasetools/sign_target_files_apks.py
index 2b45825..5867c6f 100755
--- a/tools/releasetools/sign_target_files_apks.py
+++ b/tools/releasetools/sign_target_files_apks.py
@@ -83,9 +83,8 @@
 
   --replace_verity_public_key <key>
       Replace the certificate (public key) used for verity verification. The
-      key file replaces the one at BOOT/RAMDISK/verity_key (or ROOT/verity_key
-      for devices using system_root_image). It expects the key filename WITH
-      the extension (e.g. verity_key.pub).
+      key file replaces the one at BOOT/RAMDISK/verity_key. It expects the key
+      filename WITH the extension (e.g. verity_key.pub).
 
   --replace_verity_keyid <path_to_X509_PEM_cert_file>
       Replace the veritykeyid in BOOT/cmdline of input_target_file_zip
@@ -555,8 +554,6 @@
     # Sets this to zero for targets without APK files, e.g., gki_arm64.
     maxsize = 0
 
-  system_root_image = misc_info.get("system_root_image") == "true"
-
   for info in input_tf_zip.infolist():
     filename = info.filename
     if filename.startswith("IMAGES/"):