get rid of fs_value_is_filename_empty

Its behaviour is identical to that of fs_value_is_filename.
It makes no sense, anyway - LOOKUP_EMPTY affects nothing
whatsoever once the pathname has been imported from userland.
And both fs_value_is_filename and fs_value_is_filename_empty
carry an already imported pathname.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>