API generation: Fix import list

[sbrossie]

The code generation of the java models and apis will create files that have unused import statement.

This is mostly due to the fact that we transform some model on the fly -- for instance a type List<Account> will be transform to Accounts. The parser knows how to automatically import the new type Accounts, but does not know whether it should remove Account -- could be used by other pieces of the apis in the file.