Refactor Spree::Api::ApiHelpers.required_fields_for

On line 36 the method is currently using '_validators'
which is a private method, so I refactored it to use
the public method 'validators' and pulled the attribute
from there.

I also simplified the deletion of two attributes into
a single line.  I was also thinking of using symbols
instead of strings but I wasn't sure if this would
break functionality somewhere else although tests
were green.