Revert "allow usage of a closure for the `code` option of fields"

This reverts commit 57e929e4eb023beaad154b122705e177a6b6f064.

Conflicts:
	Admin/BaseFieldDescription.php
Revert "optimization to call `$this->getOption('code')` only once"

This reverts commit 451616fa6550199c1d089de9ba9952556e6c8f5e.

Conflicts:
	Admin/BaseFieldDescription.php