The detection sometimes leads to errorneous warnings.
We need to improve it later. For now I just remove the
detection as it doesn't fully work anyway.
The commit introduces the following changes:
* Now there is no need to pass validating function in
ejabberd_config:get_option() functions, because the configuration
keeps already validated values.
* New function ejabberd_config:get_option/1 is introduced
* Function ejabberd_config:get_option/3 is deprecated. If the function
is still called, the second argument (validating function) is simply
ignored.
* The second argument for ejabberd_config:get_option/2 is now
a default value, not a validating function.
