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.

Add CLEAN sections to mysqli and PDO mysql tests

Closes GH-6756.

Fixed bug #63185