Fix "make distcheck" by distributing doc/ni.texi,

even though it's generated.  Reviewed by John Darrington.
(doc_pspp_TEXINFOS): Add doc/ni.texi.
(nodist_doc_pspp_TEXINFOS): Remove.
(am__TEXINFO_TEX_DIR) Removed (was just a kluge anyhow).
($(INFO_DEPS)): Removed.
($(HTML_DEPS)): Removed.
(CLEANFILES): Removed.