Remove better errors hook

We have our own middleare reporting errors to the API. Monkey patching
better_errors in the practice the behavior is just reporting the errors
twice: one by our middleware and another one by better_errors.

Beside this, depending on the better_errors version we have two
different behaviors:

- versions < 2.0: the error is reported twice
- versions >= 2.0: we report one error by our middleware and a 'Empty
- message' error cause the exception managed by better_errors doesn't
- inherit `Exception` and it's a custom object by better_errors.

I'm not sure why we had this monkey patch but seems it's not useful and
in any scenario is becoming finally in two reports by exception.

lib/rollbar.rb

@@ -776,7 +776,6 @@ def require_hooks
       require 'rollbar/goalie' if defined?(Goalie)
       require 'rollbar/rack' if defined?(Rack)
       require 'rollbar/rake' if defined?(Rake)
-      require 'rollbar/better_errors' if defined?(BetterErrors)
     end
 
     def require_core_extensions