Revert "Change AccessConditionDefinition in the MODS model so that it descends from XsString rather than ExtensionDefinition, and ensure its attributes are consistent with the MODS schema. This fixes that the <accessCondition> element would not accept text input in the crosswalk interface (#119)."

This reverts commit 9fa6cbe593d6c8f8d971a93f447bb236ac933e9a.