releasetools: Deprecate GKI build rules

We no longer build GKIs from the platform tree.
These build commands were neither used nor maintained anymore, so clean
them up to reduce maintenance effort.

Bug: 229701033
Fix: 229701033
Test: presubmit
Change-Id: Ie882fccd864920289e48366e99a4ebd67e784d0d
diff --git a/tools/releasetools/validate_target_files.py b/tools/releasetools/validate_target_files.py
index 82b3107..84a2f7e 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, e.g., gki_arm64, gki_x86_64, etc., are system.img-less.
+  # Some targets are system.img-less.
   if 'IMAGES/system.img' in input_zip.namelist():
     CheckAllFiles('system')