Backpack docs: merge backpack-shaping into algorithm, sigs no longer provide

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

docs/backpack/Makefile

-all: backpack-impl.pdf backpack-manual.pdf ubackpack.pdf backpack-shaping.pdf algorithm.pdf
+all: backpack-impl.pdf backpack-manual.pdf ubackpack.pdf algorithm.pdf
 
 ubackpack.pdf: ubackpack.tex
 	latexmk -pdf -latexoption=-halt-on-error -latexoption=-file-line-error -latexoption=-synctex=1 ubackpack.tex || ! rm -f $@
@@ -9,8 +9,5 @@ backpack-impl.pdf: backpack-impl.tex
 backpack-manual.pdf: backpack-manual.tex
 	latexmk -pdf -latexoption=-halt-on-error -latexoption=-file-line-error -latexoption=-synctex=1 backpack-manual.tex || ! rm -f $@
 
-backpack-shaping.pdf: backpack-shaping.tex
-	latexmk -pdf -latexoption=-halt-on-error -latexoption=-file-line-error -latexoption=-synctex=1 backpack-shaping.tex || ! rm -f $@
-
 algorithm.pdf: algorithm.tex
 	latexmk -pdf -latexoption=-halt-on-error -latexoption=-file-line-error -latexoption=-synctex=1 algorithm.tex || ! rm -f $@