Makefile: do not touch the git objects with make clean

I've been avoiding `make clean` for a long time due to its eagerness to kill
all the git objectfiles.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

Makefile

@@ -121,7 +121,6 @@ uninstall:
 
 clean:
 	rm -f cgit VERSION *.o *.d
-	cd git && $(MAKE) clean
 
 distclean: clean
 	git clean -d -x