Remove rake task check_parsing_by_psych
This patch removes the Rake task "check_parsing_by_psych".
In Ruby 1.8, Syck was used as a YAML library, and in Ruby 1.9, Psych was added and the two YAML libraries coexisted. Syck was later removed in Ruby 2.0, and now only Psych is available.
The Rake task check_parsing_by_psych was added in the process of making Redmine compatible with Ruby 1.9 (#8847), to check that YAML translation files can be parsed without errors in Psych as well as Syck.
However, Syck was removed in Ruby 2.0 and now Pysch is always used to parse when running the rake task update_locales, so check_parsing_by_psych is no longer needed.
For these reasons, I propose to remove check_parsing_by_psych.