revert dummy log device commit

revert https://github.com/fluent/fluentd/commit/b24184bf444e3753fc9df0df0e6325abe93ed5db because tests of my plugins fail. 

See https://travis-ci.org/sonots/fluent-plugin-elapsed-time/jobs/25352227, or
https://travis-ci.org/sonots/fluent-plugin-measure_time/jobs/25350912. The test is failing at https://github.com/fluent/fluentd/blob/550e1f4553df2100d0de351d4d33baafd87ecf11/lib/fluent/plugin.rb#L72 because `$log` is nil. 

Please let me know if there are better ways to fix this problem.