refactor: remove PHPStan callable signature baseline

[memleakd]

Description
Adds PHPDoc callable signatures where PHPStan reported missing callable parameter/return signatures.

This removes all entries from missingType.callable.neon, deletes the now-empty baseline file, and removes its loader include. It also removes one related assign.propertyType baseline entry for Table::$function, since the property PHPDoc now supports the invalid callback string tested in TableTest.

Related to #6310.

Verification:

• composer phpstan:check
• git diff --check

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[paulbalandan]

This needs rebasing too but wait until the other cleanup is merged to reduce duplicate effort. Thanks!