Fix concurrency issue with multiline codec and other grok-based plugins

[edmocosta]

Fixed a concurrency issue with multiline codec and other grok-based plugins, that when combined, was generating the following error:

{:action=>LogStash::PipelineAction::Create/pipeline_id:pipeline2, :exception=>"Java::JavaLang::IllegalStateException", :message=>"Unable to configure plugins: (LoadError) load error: cabin/mixins/logger -- java.lang.StringIndexOutOfBoundsException: Index -1 out of bounds for length 16"}

[edmocosta]

This issue was being caused by jruby/jruby#8960, and this PR is no longer needed.