init: remove the existing early_mount code

keeps parts of the code that are still needed for the fs_mgr
+ dt based implementation

b/27805372

Test: boot angler, sailfish without regressions

Change-Id: I1b08f8b7b4f2e67118d328443a5011c0f5ead919
Signed-off-by: Sandeep Patil <sspatil@google.com>