remove custom transport proc instructions as they are no longer supported

[ekump]

2.0 Upgrade Guide notes
upgrade guide should already call out that custom transport procs are removed from 2.0

What does this PR do?
Update dev guide doc to remove instructions on how to implement a custom transport proc.

Motivation:
Support for custom transport procs has been removed in 2.0

Additional Notes:

How to test the change?
Doc change only

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!

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-02-26 20:19:56

Comparing candidate commit 936dcec in PR branch ekump/remove-transport-proc-info-from-dev-guide with baseline commit d24fd06 in branch 2.0.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 7 metrics, 1 unstable metrics.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.07%. Comparing base (d24fd06) to head (936dcec).

Additional details and impacted files

@@            Coverage Diff             @@
##              2.0    #3487      +/-   ##
==========================================
- Coverage   98.07%   98.07%   -0.01%     
==========================================
  Files        1235     1235              
  Lines       71980    71980              
  Branches     3393     3393              
==========================================
- Hits        70595    70594       -1     
- Misses       1385     1386       +1     

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