BootReceiver: Remove error log when last-header.txt is missing

It's normal for /data/system/last-header.txt to not exist on
the first boot after factory reset. The code already handles
this case properly so the Slog.e is just noise that shows up
in bugreports.

Bug: 37513804
Test: manual - boot after factory reset, check logcat
Change-Id: I1c8e3437cdbda5e04c0299764c7e4921bef67cb3