Remove ddtrace transport constants

[marcotc]

2.0 Upgrade Guide notes
The module Datadog::Transport::Ext::HTTP has been removed, use Datadog::Core::Transport::Ext::HTTP instead.
The module Datadog::Transport::Ext::Test has been removed, use Datadog::Core::Transport::Ext::Test instead.
The module Datadog::Transport::Ext::UnixSocket has been removed, use Datadog::Core::Transport::Ext::UnixSocket instead.

Motivation:
This module has been moved, but the an old "alias" for it was left behind for backwards compatibility.

How to test the change?
All tests should pass after removing this unused code.

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 (4dbbdb9) 98.06% compared to head (03b73be) 98.06%.

Additional details and impacted files

@@            Coverage Diff             @@
##              2.0    #3336      +/-   ##
==========================================
- Coverage   98.06%   98.06%   -0.01%     
==========================================
  Files        1241     1241              
  Lines       71830    71830              
  Branches     3386     3386              
==========================================
- Hits        70438    70437       -1     
- Misses       1392     1393       +1     

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