init: remove the class_{start,reset}_post_data commands

Remove the class_start_post_data and class_reset_post_data commands,
since they aren't used anymore.  They were only used on devices that
used FDE (Full Disk Encryption), via actions in rootdir/init.rc.  These
actions have been removed, since support for FDE has been removed.
There is no use case for these commands in vendor init scripts either.

Keep the mark_post_data command, since DoUserspaceReboot() uses the
post-data service flag even on non-FDE devices.

Bug: 191796797
Change-Id: Ibcd97543daa724feb610546b5fc2a0dd7f1e62e7