releasetools: Delete the obsolete --aslr_mode/-a flag.

It was added in commit 96be7205
("Working ASLR implementation.") in 2010, and removed in commit
1807e700 ("don't generate retouch
commands in OTA scripts") in 2012.

Remove the obsolete --aslr_mode flag.

Test: ota_from_target_files.py still works (by generating incremental
      and full OTAs respectively).

Change-Id: I6d8e62730ac192f3574d484c4a4b9b43b4ee0a9e