Revert "LU-4933 osc: Automatically tune the max_dirty_mb"

This causes frequent test failures in conf-sanity.sh
test_76a() due to changing max_dirty_mb automatically.

This reverts commit 5303c139e3cef8ed09b85cf828afbbe24ea0d7db.

Change-Id: I013d75ac85010262419a57575552bf55eff9fcdd
Reviewed-on: http://review.whamcloud.com/10402
Reviewed-by: Patrick Farrell <paf@cray.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>