tcp: remove the transport layer reference counting mechanism

[gdamore]

The upper SP layer takes care not to destroy and endpoint while it has outstanding pipes.

It's possible that the dueling reference counts between the upper and lower layers have been responsible for some hangs on shutdown sometimes seen with other transports.

fixes #

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[codecov]

Codecov Report

Attention: Patch coverage is 91.30435% with 2 lines in your changes missing coverage. Please review.

Project coverage is 81.29%. Comparing base (c3620e6) to head (f6ddada).

Files with missing lines	Patch %	Lines
src/sp/transport/tcp/tcp.c	91.30%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1948      +/-   ##
==========================================
- Coverage   81.59%   81.29%   -0.31%     
==========================================
  Files          94       94              
  Lines       24010    23988      -22     
  Branches     3199     3194       -5     
==========================================
- Hits        19591    19500      -91     
- Misses       4347     4416      +69     
  Partials       72       72              

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