dev/core#1645 fix regression by removing form inheritence

dev/core#1645 fix  regression  by removing form inheritence

The Assign  Account form is failing to load because it inherits indirectly from CRM_Admin_Form which
now has a different visibility on the _id property.

I  took a look and there really is no reason for this complex  inheritence - this is a standalone form
and the setDefaults of  the parent seems of no use, let alone the parent's parent.

Assign, edit & browse seem to still work fine