gtk-doc: don't manually call gtkdocize

gtk-doc: don't manually call gtkdocize

The autoreconf call will now call gtkdocize if needed, so we don't need
to run it manually here.

This obsoletes GTKDOC_DOCDIR.  If this is needed then the replacement is
to ensure that the configure.ac has a call to GTK_DOC_CHECK which passes
--docdir.

For example, this is the change required for kmod:

-GTK_DOC_CHECK([1.14],[--flavour no-tmpl-flat])
+GTK_DOC_CHECK([1.14],[--flavour no-tmpl-flat --docdir libkmod/docs])

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>