Fix recursive require in Railtie

[marcotc]

Fixes #3340

What does this PR do?
This PR removes a redundant require on the DatadogAutoInstrumentRailtie class.
It's not possible to run this code today without the required file already being loaded.

How to test the change?

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d809f33) 98.23% compared to head (307c740) 98.24%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3365   +/-   ##
=======================================
  Coverage   98.23%   98.24%           
=======================================
  Files        1254     1254           
  Lines       73217    73217           
  Branches     3430     3430           
=======================================
+ Hits        71928    71931    +3     
+ Misses       1289     1286    -3     

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