Remove duplicate entries in EXTENSIONS

These would cause failures if the extensions is loaded dynamically.

Port skipif.inc files to EXTENSIONS

Migrate more SKIPIF -> EXTENSIONS (#7139)

This is a mix of more automated and manual migration. It should remove all applicable extension_loaded() checks outside of skipif.inc files.

Fixed bug #70066

If we fall back to emulated prepared statements, destroy S->stmt,
so the code doesn't get confused about which mode we're in.