fs_mgr_fstab: Remove legacy DSU logic for Q first_stage_init

Deprecate DSU booting support on Q first_stage_init.
Now kDsuActiveFile and kGsiLpNamesFile must be accessible for
ReadFstabFromFile() to return successfully.

Bug: 235111004
Test: Presubmit GSI boot test
Change-Id: I5760eb9aa05610085dd7b7b78940f22fba425229