Remove method that overrides Object#equal? (cleanup)

From the docs:

> Unlike ==, the equal? method should never be overridden by subclasses
> as it is used to determine object identity (that is, a.equal?(b) if
> and only if a is the same object as b)

sources:
http://www.ruby-doc.org/core-1.9.3/BasicObject.html#method-i-equal-3F
http://www.ruby-doc.org/core-2.1.2/BasicObject.html#method-i-equal-3F
Merge pull request #188 from gkop/dont-override-object-equal

Remove method that overrides Object#equal? (cleanup)