Replace OPENSSL_NO_TLS_PHA with SSL_VERIFY_POST_HANDSHAKE

[WillChilds-Klein]

Notes

PR #1526 introduced the OPENSSL_NO_TLS_PHA directive mostly for the purposes of AWS-LC's compatibility with CPython, but in cpython PR #117785 @encukou points out that detecting the absence of OpenSSL's own SSL_VERIFY_POST_HANDSHAKE directive is sufficient. This change removes AWS-LC's OPENSSL_NO_TLS_PHA directive in favor of detecting absence of SSL_VERIFY_POST_HANDSHAKE.

Testing

• existing CI integration tests

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.22%. Comparing base (b2bbd9c) to head (2fbd20b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1668   +/-   ##
=======================================
  Coverage   78.22%   78.22%           
=======================================
  Files         566      566           
  Lines       95193    95193           
  Branches    13665    13663    -2     
=======================================
+ Hits        74465    74466    +1     
  Misses      20133    20133           
+ Partials      595      594    -1     

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