remount: Remove AVB 1.0 code & opaque exit code

* Remove AVB 1.0 (fec).
* Assert device is bootloader unlocked in main().
* Since error is already logged to stderr and logd, there is no need to
  return an opaque enum value as error code. Just return 1 if main()
  encounters any error.

Bug: 241688845
Test: Presubmit
Test: adb-remount-test
Change-Id: I06df6f92a3d4adaca77061920736056c9051c112