WL#7149: Remove innodb_file_io_threads variable in 5.7

Affects: Server-5.7   —   Status: Complete   —   Priority: Medium

The innodb_file_io_threads system variable was deprecated in 5.5, all traces of
it can be removed from the code in 5.7.
The internal variable is used for tracking the total number of IO threads.
Remove the code that allows user to set its value.