Revert "releasetools: Deprecate GKI build rules"

This reverts commit faaede0f017a95889e33251b0258b52caaba3113.

Reason for revert: caused GSI sign failure b/311134118#comment23

Change-Id: Id794a8ab3369c907834a73cc88ea1640dc7492d1
diff --git a/tools/releasetools/validate_target_files.py b/tools/releasetools/validate_target_files.py
index 84a2f7e..82b3107 100755
--- a/tools/releasetools/validate_target_files.py
+++ b/tools/releasetools/validate_target_files.py
@@ -132,7 +132,7 @@
     return
 
   # Verify IMAGES/system.img if applicable.
-  # Some targets are system.img-less.
+  # Some targets, e.g., gki_arm64, gki_x86_64, etc., are system.img-less.
   if 'IMAGES/system.img' in input_zip.namelist():
     CheckAllFiles('system')