make: Do not delete deployable-lightning.pdf on maintainer-clean

It is not part of the default documentation generation, thus will show up in
the git diff every time a maintainer-clean is done. We could also add it to
the default documentation generation, however it requires some rather
heavy-weight latex dependencies, and the file hasn't changed in several years.

doc/Makefile

@@ -100,7 +100,6 @@ check-manpages: all-programs
 	@tools/check-manpage.sh "lightningd/lightningd --lightning-dir=/tmp/" doc/lightningd-config.5.md
 
 doc-maintainer-clean:
-	$(RM) doc/deployable-lightning.pdf
 	$(RM) $(MANPAGES)
 
 doc-clean: