Removing obsolete PostDataDecorator#build_byline.

Method no longer used; incorporated into `PostEntity`.

RSpec: 1072 examples, 0 failures, 33 pending
RuboCop: 249 files inspected, no offenses detected`
UserPostSelector no longer does Pundit, UserData

Nor does it have any remaining dependency on the helper/view context.

RSpec: 1072 examples, 0 failures, 32 pending
RuboCop: 249 files inspected, no offenses detected