[pigeon] remove runtime type/nullability checks in deserialization in all generators #116972
Description
Currently there are some sporadic runtime checks for type and nullability that are only verifying that the generated code has the correct types.
This slows down client performance slightly and shouldn't be needed as pigeon should be verified to provide typing correctly before publishing.
As mentioned here flutter/packages#2745 (comment)
/cc @stuartmorgan