Allows user code to retain references to memoized helpers

Created by: JoshCheek

Accomplish this by no longer clearing the example's ivars.

Fixes rspec/rspec-core#1921

Context

• Originally clearing ivars due to memory leak: https://github.com/rspec/rspec-core/issues/321
• Threadsafe memoized helpers caused __memoized to stop lazily initializing: https://github.com/rspec/rspec-core/issues/321
• This caused it to be permawiped by by the resetting of the example's ivars: https://github.com/rspec/rspec-core/issues/1921

However, this patch tests the behaviour of the memory leak, rather than its mechanics, which shows that it was fixed at some point. So we simply remove that code.