build: Remove intermediate PDF files from EXTRA_DIST.

* doc/Makefile.am (EXTRA_DIST): Remove PDF files.
(BUILT_SOURCES): Likewise.

When texi2pdf is invoked, etex is invoked. And it's actually pdftex
these days, which can process PNG files directly. So, no need to
prepare PDF files.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>