fs_mgr: removing fs_mgr_read_fstab_with_dt()

Currently there is no client requires fs_mgr_read_fstab_with_dt() so
remove this API. Devices with fstab entries in device tree should use
fs_mgr_read_fstab_default() to get default fstab, or just use
fs_mgr_read_fstab_dt() to get those in device tree.

Bug: 35811655
Test: normal boot sailfish
Test: run factory data reset in sailfish
Test: recovery boot sailfish
Test: adb sideload in sailfish

Change-Id: I1579d81cea7366cf3867cfe5ad56feb36cbb7246