The master-bin.info file records the binlog state across normal server
restarts (not crashes) to avoid the overhead of scanning last binlog file at
server start.

But we should delete the file as soon as it is read so there is no risk of
errorneously (in server code or by user) reading out-of date information from
it (eg. RESET MASTER for example).

In addition, it might be useful to read the file (if present) at server start
even if binlog files are gone (this allows to recover the binlog state even if
binlog files are intentionally omitted from a cold backup).