Don't change visibility based on if YARD is loaded.

Originally, I expected YARD to be loaded only when
we generate docs, but users may have YARD loaded
in their environment as well.

Fixes #473.
Remove spec.

It doesn't really make sense to keep a spec that
specifies behavior when a specific gem is loaded
when there's nothing left in the project that is
custom code when that gem is loaded.

I thought it was worth committing the spec for the
history, though, so I kept it in the last commit.
Merge pull request #474 from rspec/fix-issue-473

Fix issue 473