Remove support for converting from FDE to FBE

Devices that launched with Android 10 or later require FBE (File Based
Encryption) from the beginning, so there's no need to support converting
to FBE after the fact anymore.  This was only ever a developer option,
so it probably wasn't used much.  And in any case, it's not used
anymore, as isConvertibleToFBE() is hard-coded to return false.  Besides
the fact that FBE has been required for several releases now, this
functionality was only ever available on devices that use FDE (Full Disk
Encryption), but FDE support has been removed from Android.

Therefore, remove this unused code.

Bug: 208476087
Change-Id: Ia765d6827e6144153c4e2a253b146ef2c75ca6fe