build: Don't require ImageMagick anymore for building document.

* configure.ac (CONVERT): Remove.
* doc/Makefile.am (images_eps): Remove.
(CLEANFILES, BUILT_SOURCES): Remove images_eps.
(.png.eps, .png.pdf): Remove suffix rules.
(dist-hook): Remove.

We only support making scute.pdf (not scute.ps any more).
Note that pdftex can handle png files directly to produce PDF.

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