Form property #DANGEROUS_SKIP_CHECK removed in D6

note, documentation for properly altering forms and avoiding the 'illegal choice' error is not readily available anywhere.

#DANGEROUS_SKIP_CHECK was initially created because altering certain form elements can be quite difficult with the options consistency being checked in _form_validate(). it becomes next to impossible to alter elements without running into the above error. and the correct way to do it was never documented, hence the birth of #DANGEROUS_SKIP_CHECK. now they've removed the hackish option without providing documentation on the proper way to alter these form elements.