Make figs includable in main document

Remove the capability to build them as standalone document but we can
include them in the main document.  It should simplify things down the
road.

book/figures/fig-inverted-tray-method.tex

@@ -1,6 +1,3 @@
-\input{./vars}
-
-\begin{document}
 \begin{tikzpicture}[node distance = 3cm, auto]
   \node [block] (init) {\footnotesize Place water tray and stone in oven};
   \node [block, right of=init] (heat_oven) {\footnotesize Heat oven to 230°C (446°F) for 30 minutes};
@@ -22,4 +19,3 @@
   \path [line] (bake) -- (remove_steam);
   \path [line] (remove_steam) -- (finish);
 \end{tikzpicture}
-\end{document}