Remove Safari UTF-8 BOM hack. Close #93

I guess this hack is no longer needed.

Amon2::Plugin::Web::JSON output's BOM for safari. This feature is taken
from @miyagawa's Catalyst::View::JSON.

The feature was introduced at miyagawa/catalyst-view-json@9b56bcf. No
reason was wrote at here.

And disabled for chrome by this commit.
miyagawa/catalyst-view-json@dbd8b6e

I guess this hack has no reason today. It's time to remove this.