[BABEL] Insert exec pr5 code cleanup

[monk0707]

Description

Currently, Babelfish carries two parallel implementations of INSERT ... EXECUTE, gated by the babelfishpg_tsql.enable_new_insert_exec GUC.
The original path goes via a tuple store based approach. The redesigned path introduced in PR0–PR4 (#4780–#4847) works on a temp table based approach.

The cleanup removes the enable_new_insert_exec GUC declaration, and the legacy insert-exec tuple store path.

Related engine PR : babelfish-for-postgresql/postgresql_modified_for_babelfish#771

Issues Resolved

Final cleanup of the INSERT-EXEC redesign stack (PR0–PR4: #4780, #4781, #4782, #4783, #4847).

Test Scenarios Covered

• Use case based — Existing INSERT-EXEC JDBC suites pass unchanged: stored-procedure INSERT EXEC, EXEC sp_executesql INSERT EXEC, dynamic-SQL INSERT EXEC, cross-database INSERT EXEC, INSERT EXEC into table variables, INSERT EXEC inside TRY/CATCH, and INSERT EXEC with explicit column lists.
• Boundary conditions — Nested INSERT EXEC, column-count mismatches between procedure output and target, and INSERT EXEC where the target relation is concurrently dropped/modified all behave as before the cleanup.
• Negative test cases — COMMIT and ROLLBACK inside an INSERT EXEC statement still raise the expected errors . INSERT EXEC inside a user-defined function still fails parser validation.
• Minor / Major version upgrade tests — No catalog or extension SQL changes; upgrades are unaffected.
• Performance tests — No performance-sensitive code path changed; only the dead branch of pre-existing runtime checks is removed.
• Tooling impact — babelfishpg_tsql.enable_new_insert_exec is removed from pg_settings. Any tooling or test harness that explicitly sets this GUC will receive an "unrecognized configuration parameter" error and must drop the setting.
• Client tests — Full JDBC test suite passes locally on the cleanup branch.

Check List

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