Remove deprecated Headers::Ext

[marcotc]

2.0 Upgrade Guide notes
Deprecated constants at Datadog::Tracing::Distributed::Headers::Ext have been removed. These constants have been moved to Datadog::Tracing::Distributed::Datadog and Datadog::Tracing::Distributed::B3.

Motivation:

Datadog::Tracing::Distributed::Headers::Ext was a dumping ground for all distributed headers. The constants have been moved to their respective propagation styles instead.

Additional Notes:

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 (1b46fe7) 97.99% compared to head (99aa0f1) 97.99%.

Additional details and impacted files

@@           Coverage Diff           @@
##              2.0    #3300   +/-   ##
=======================================
  Coverage   97.99%   97.99%           
=======================================
  Files        1248     1248           
  Lines       72058    72058           
  Branches     3401     3401           
=======================================
  Hits        70616    70616           
  Misses       1442     1442           

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