Remove extra docdir variable from Makefile.am

The docdir variable is defined by autoconf, so we don't need to define
it ourselves. This way, the target documentation directory can
be changed via ./configure instead of patching Makefile.am.

In truth, we should install the pdf and html files using the pdf and
html targets, since configure defines pdfdir=htmldir=docdir; but
installing them in docdir is good enough.

Reviewed-by: imsnah