thread_concurrency

The system variable 'thread_concurrency' has been
      (re-)enabled on all platforms, to prevent startup
      errors.
      
      'thread_concurrency' is unused and has no effect,
      on any platform, in MySQL 5.1 and later versions.  It 
      will be deprecated, and removed, in context of 
      worklog WL#5265
     @ mysql-test/include/have_thread_concurrency.inc
        include/have_thread_concurrency.inc was only needed
        to support platforms that didn't have the
        thread_concurrency variable available.  Since the
        variable is now available on all platforms, the file
        is removed.