fs_mgr: remove atgc mount option for zoned device

F2FS gives EINVAL when trying to mount zoned device with atgc. This patch
allows to keep the single fstab to support legacy and zoned devices at the
same time.

Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>
Change-Id: I47a667443e7e60cb6729553b2ca24026e21fd90d