Remove tuple-store INSERT EXEC implementation

[monk0707]

Description

This PR removes the old tuple-store based INSERT EXEC implementation from the engine side. The INSERT EXEC feature has been reimplemented using a DestReceiver-based approach in the extension side.

Changes:

• Remove called_from_tsql_insert_exec_hook_type typedef from src/include/access/attmap.h
• Remove called_from_tsql_insert_exec_hook extern declaration from attmap.h
• Remove called_from_tsql_insert_exec_hook variable from src/backend/access/common/attmap.c
• Remove hook usage from attmap.c type checking logic
• Remove hook usage from src/backend/access/common/tupconvert.c value casting logic
• Remove entire old tuple-store based INSERT EXEC implementation from src/backend/executor/nodeModifyTable.c (~60 lines)

Why:
The new DestReceiver-based approach:

1. Uses a temp table to store results instead of tuple store
2. Handles type coercion properly
3. Provides better error handling and transaction semantics

The old engine-side hooks are no longer needed.

Related Extension PR: babelfish-for-postgresql/babelfish_extensions#4856

Issues Resolved

Cleanup of deprecated INSERT EXEC engine hooks as part of the DestReceiver-based INSERT EXEC reimplementation.

Check List

• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is under the terms of the PostgreSQL license, and grant any person obtaining a copy of the contribution permission to relicense all or a portion of my contribution to the PostgreSQL License solely to contribute all or a portion of my contribution to the PostgreSQL open source project.

For more information on following Developer Certificate of Origin and signing off your commits, please check here.