Remove unwrap_object.

There is no good reason to have both unwrap_object and unwrap_jsmanaged.
Removing unwrap_object simplifies the codegen and makes further
simplifications easier.
auto merge of #2126 : Ms2ger/servo/avoid-unwrap_object, r=jdm

There is no good reason to have both unwrap_object and unwrap_jsmanaged.
Removing unwrap_object simplifies the codegen and makes further
simplifications easier.