The only purpose of the field FIL_PAGE_FILE_FLUSH_LSN was to
store the log sequence number for a new ib_logfile0 when the
InnoDB redo log was missing at startup.

Because FIL_PAGE_FILE_FLUSH_LSN no longer serves any purpose,
we will stop updating it. The writes of that field were inherently
risky, because they were not covered by neither the redo log nor
the doublewrite buffer.

Warning: After MDEV-14425 and before this change, users could perform
a clean shutdown of the server, replace the ib_logfile0 with a
0-length file, and expect a valid log file to be created on the
next server startup. After this change, if the FIL_PAGE_FILE_FLUSH_LSN
had ever been updated in the past, the server would still create a
log file in such a scenario, but possibly with an incorrect (too small)
LSN. Users should not manipulate log files directly!