Remove 'utc' field from TimeSlicedOutput conf to prevent ConfigError in plugin helpers

[repeatedly]

Helpers/Compat utils can't judge user specifies both localtime/utc or converted result.
This changes conf dumpped result but it seems acceptable.

Without this change, v0.12 TimeSlicedOutput plugins with utc parameter don't work.

[repeatedly]

This is incomplete. Incompatibility still exists by conf['timezone'] = "+0000".
v0.12 formatter doesn't generate "+0000" suffix time with utc.
Existing plugins assume formatted result is "Z" suffix time with utc.

[repeatedly]

@tagomoris Could you handle this problem? Currently, v0.14 has incompatibility issue with v0.12 TimeSlicedOutput.
S3 plugin has tests for thsese cases, so passed S3 tests may cover other TimeSlicedOutput plugin cases.

Here is one failed case:

Failure: test_format(S3OutputTest)
/Users/repeatedly/dev/fluentd/fluentd/lib/fluent/test/output_test.rb:139:in `block in run'
/Users/repeatedly/dev/fluentd/fluentd/lib/fluent/test/input_test.rb:124:in `block in run'
/Users/repeatedly/dev/fluentd/fluentd/lib/fluent/test/base.rb:71:in `run'
/Users/repeatedly/dev/fluentd/fluentd/lib/fluent/test/input_test.rb:123:in `run'
/Users/repeatedly/dev/fluentd/fluentd/lib/fluent/test/output_test.rb:124:in `run'
test/test_out_s3.rb:151:in `test_format'
     148:     d.expect_format %[2011-01-02T13:14:15Z\ttest\t{"a":1}\n]
     149:     d.expect_format %[2011-01-02T13:14:15Z\ttest\t{"a":2}\n]
     150: 
  => 151:     d.run
     152:   end
     153: 
     154:   def test_format_included_tag_and_time
<"2011-01-02T13:14:15Z\ttest\t{\"a\":1}\n2011-01-02T13:14:15Z\ttest\t{\"a\":2}\n"> expected but was
<"2011-01-02T13:14:15+00:00\ttest\t{\"a\":1}\n2011-01-02T13:14:15+00:00\ttest\t{\"a\":2}\n">

[repeatedly]

@tagomoris I added test for this case. Now, test failed.

Failure: test: check formatted time compatibility with utc. Should Z, not +00:00(FluentOutputTest::TimeSlicedOutputTest::test emit)
/Users/repeatedly/dev/fluentd/fluentd/lib/fluent/test/output_test.rb:139:in `block in run'
/Users/repeatedly/dev/fluentd/fluentd/lib/fluent/test/input_test.rb:124:in `block in run'
/Users/repeatedly/dev/fluentd/fluentd/lib/fluent/test/base.rb:71:in `run'
/Users/repeatedly/dev/fluentd/fluentd/lib/fluent/test/input_test.rb:123:in `run'
/Users/repeatedly/dev/fluentd/fluentd/lib/fluent/test/output_test.rb:124:in `run'
/Users/repeatedly/dev/fluentd/fluentd/test/test_output.rb:261:in `block (2 levels) in <class:TimeSlicedOutputTest>'
     258:         time = Time.parse("2016-11-08 12:00:00 UTC").to_i
     259:         d.emit({"a" => 1}, time)
     260:         d.expect_format %[2016-11-08T12:00:00Z\ttest\t{"a":1,"time":"2016-11-08T12:00:00Z"}\n]
  => 261:         d.run
     262:       end
     263:     end
     264:   end
<"2016-11-08T12:00:00Z\ttest\t{\"a\":1,\"time\":\"2016-11-08T12:00:00Z\"}\n"> expected but was
<"2016-11-08T12:00:00+00:00\ttest\t{\"a\":1,\"time\":\"2016-11-08T12:00:00+00:00\"}\n">

[tagomoris]

I pushed two commits to fix this problem.
@repeatedly Could you check these and merge if these are ok?

[repeatedly]

I tested the patch with s3 plugin test and it fixes broken tests.
Thanks for quick fix.

[repeatedly]

And LGTM