doc: speed "noop" compilation times by not force-building markdown an…

…d man pages every time.

Their dependencies seem correct, so we shouldn't have to build them every time.

Before:
```shell
$ time make -s
real	0m5.183s
user	0m5.134s
sys	0m11.715s
```

After:

```shell
$ time make -s
real	0m0.784s
user	0m0.775s
sys	0m1.159s
```

Signed-off-by: Rusty Russell <[email protected]>

doc/Makefile

@@ -188,9 +188,6 @@ endif
 $(MANPAGES): doc/%: doc/%.md tools/md2man.sh version_gen.h
 	@VERSION=$(VERSION) tools/md2man.sh $(LOWDOWN) $< > $@
 
-$(MANPAGES): $(FORCE)
-$(MARKDOWN_WITH_SCHEMA): $(FORCE)
-
 doc/protocol-%.svg: test/test_protocol
 	test/test_protocol --svg < test/commits/$*.script > $@