The option binlog_optimize_thread_scheduling was initially added
to provide a safe alternative for the newly added binlog group
commit logic, such that when 0, it would disable a leader thread
from performing the binlog write for all transactions that are a
part of the group commit. Any problems related to the binlog group
commit optimization should be sorted out by now, so we can
deprecate-to-eventually-remove the option altogether.

This commit performs the deprecation, and the removal is tracked
by MDEV-33745. Note, as the option is only able to be provided
via configuration at startup time, users will not see a
deprecation message unless looking through the CLI help
message.