merge-recursive: remove return value from get_files_dirs

merge-recursive: remove return value from get_files_dirs

The return value of the get_files_dirs call is never being used.
Looking at the history of the file and it was originally only
being used for debug output statements.  Also when
read_tree_recursive return value is non zero it is changed to
zero.  This leads me to believe that it doesn't matter if
read_tree_recursive gets an error.

Since the debug output has been removed and the caller isn't
checking the return value there is no reason to keep calculating
and returning a value.

Signed-off-by: Kevin Willford <kewillf@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>