channel.c: Remove dead AST_GENERATOR_FD code.

channel.c: Remove dead AST_GENERATOR_FD code.

Nothing ever sets the `AST_GENERATOR_FD`, so this block of code will
never execute. It also is the only place where the `generate` callback
is called with the channel lock held which made it difficult to reason
about the thread safety of `ast_generator`s.

In passing, also note that `AST_AGENT_FD` isn't used either.