releasetools: Drop the support for fstab_version 1.

fstab_version is defined by RECOVERY_FSTAB_VERSION in
bootable/recovery. We have moved to fstab_version 2 since commit
f35d1cef7c19db975a1295e8c23c7fb8bd2489f9 (landed into JB MR2).

Drop the support for fstab_version 1, since we won't run the latest OTA
script over a JB target_files zip.

Test: No impact on building full/incremental OTAs.
Change-Id: Ia87c4e7da6c5e71ce0908fca2e4f1ad1c06ba869