Remove unreachable code

[michaelcameron]

This code creates a warning when consuming js-joda via the google closure compiler.

It's been years since it was reachable and the other places with a similar exception have no implementation below. I know it's ignored for eslint, but as mentioned, it causes warnings in other tool chains.

[coveralls]

Coverage increased (+0.02%) to 92.535% when pulling fbbc32a on michaelcameron:remove-unreachable-code into 767ee7e on js-joda:master.

[pithu]

Hi @michaelcameron,

Thanks for contributing, your change sounds reasonable, i could merge it right away if you fix the linter error, see https://travis-ci.org/js-joda/js-joda/jobs/455102966

/home/travis/build/js-joda/js-joda/src/Month.js
  13:9   error  'DateTimeFormatterBuilder' is defined but never used  no-unused-vars
  72:20  error  'style' is defined but never used                     no-unused-vars
  72:27  error  'locale' is defined but never used                    no-unused-vars

[michaelcameron]

Linter errors have been fixed