[#24433] YSQL: Remove flag workaround from YSQL major version upgrade…

… tests

Summary:
Commit 7dffbbc worked around a performance issue on Mac release builds caused by commit 981415e, which set pg_client_use_shared_memory to true for release builds. Now that the performance issue has been removed by commit 555af7d, revert the workaround in the YSQL major upgrade shell tests.
Jira: DB-13340

Test Plan:
Jenkins: compile only

On MacOS 14 arm64:
  ./yb_build.sh release --sj
  pg15_tests/test_upgrade.sh

Reviewers: hsunder, fizaa, telgersma

Reviewed By: hsunder

Subscribers: ybase, yql

Differential Revision: https://phorge.dev.yugabyte.com/D38978

pg15_tests/common_upgrade.sh

@@ -7,8 +7,7 @@ pghost3=127.0.0.$((ip_start + 2))
 
 # TEST_always_return_consensus_info_for_succeeded_rpc=false is needed to upgrade a release build to
 # debug.
-# On MacOS, pg_client_use_shared_memory harms initdb performance significantly.
-common_pg15_flags="TEST_always_return_consensus_info_for_succeeded_rpc=false,pg_client_use_shared_memory=false"
+common_pg15_flags="TEST_always_return_consensus_info_for_succeeded_rpc=false"
 # yb_enable_expression_pushdown=false is needed because the expression pushdown rewriter is not yet
 # implemented.
 common_tserver_flags='"ysql_pg_conf_csv=yb_enable_expression_pushdown=false"'