In registry.c:registry_clear_data, the registered data is iterated and
invoke each 'free' function with the data passed:

  for (registration = data_registry->registrations, i = 0;
       i < fields->num_data;
       registration = registration->next, i++)
    if (fields->data[i] != NULL && registration->data->free != NULL)
      adaptor (registration->data->free, container, fields->data[i]);

we can see that data is passed to function 'free' and data is not NULL.
In each usage, we don't have to get the data again through key and
do NULL pointer checking.  This patch is to simplify them.

gdb:

2013-10-29  Yao Qi  <yao@codesourcery.com>

	* auto-load.c (auto_load_pspace_data_cleanup): Get data from
	parameter 'arg' instead of from program_space_data.
	* objfiles.c (objfiles_pspace_data_cleanup): Likewise.
	* solib-darwin.c (darwin_pspace_data_cleanup): Likewise.
	* solib-dsbt.c (dsbt_pspace_data_cleanup): Likewise.
	* solib-svr4.c (svr4_pspace_data_cleanup): Likewise.
	* inflow.c (inflow_inferior_data_cleanup): Get data from
	parameter 'arg' instead of inferior_data.
	* registry.h: Add comments.