box/func: module_reload -- drop redundant argument

The only purpose of the module argument is to
notify the caller that the module doesn't exist.
Lets simplify the code and drop this argument.

Part-of #4642

Acked-by: Serge Petrenko <sergepetrenko@tarantool.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>