Incremental patch part 2

Removing dead code and changing a note level message to a warning.


sql/sql_plugin.cc:
  * Remove free_slots. The only purpose for this variable was to trigger
    a redundant warning message  and it failed.
  * Change the note level message about shutting down plugins which
    didn't end nicely to a warning level message. (If this shutdown
    fails and there still are reference counts in the plugin an
    additional error level message is emitted)