Remove unnecessary forward declarations from reactor.hh

reactor.hh includes forward declarations for input_stream, output_stream
template classes. These are unnecessary, because it #include's iostream.hh,
which contains the full definition of these classes.

These unnecessary forward declarations are harmless in the current code, but
they will become harmful if we change the definitions in iostream.hh (e.g., I
wanted to make input_stream a "final" class).

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>

core/reactor.hh

@@ -74,12 +74,6 @@ class pollable_fd;
 class pollable_fd_state;
 class lowres_clock;
 
-template <typename CharType>
-class input_stream;
-
-template <typename CharType>
-class output_stream;
-
 struct free_deleter {
     void operator()(void* p) { ::free(p); }
 };