Remove benchmark file.

This benchmark suggested that `require_relative` isn't
any faster than `require`, because it put `rspec/core/lib`
onto the load path as the first dir, which made `require`
for rspec-core files O(1) like `require_relative`.

In practice, people's load path will have many more directories
and this benchmark doesn't align with that reality.

[ci skip]

Author: myronmarston