Remove configuration for priority_sampling

[marcotc]

2.0 Upgrade Guide notes

The configuration setting c.tracing.priority_sampling has been removed. This option was used to disable priority sampling, which was enabled by default.
To disable priority sampling, you now have to create a custom sampler.

Motivation:

Priority sampling is the preferable way to store a sampling decision at Datadog.
Disabling it also disables features in the Datadog Agent and creates subpart reporting in the Ingestion Control page.

How to test the change?
There are unit and integration tests for all changes.

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 (c7ef57c) 98.06% compared to head (2d1e3c0) 98.05%.

Additional details and impacted files

@@            Coverage Diff             @@
##              2.0    #3325      +/-   ##
==========================================
- Coverage   98.06%   98.05%   -0.01%     
==========================================
  Files        1243     1243              
  Lines       71873    71808      -65     
  Branches     3388     3381       -7     
==========================================
- Hits        70481    70414      -67     
- Misses       1392     1394       +2     

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