build: remove wonky auto top-level convenience targets

These were meant to help build subdir targets from the top builddir, but cause
infinite recursion when going the other way.

If anyone actually uses these, we can add back specific targets.

Makefile.am

@@ -76,9 +76,6 @@ $(BITCOIN_WIN_INSTALLER): all-recursive
 	  echo error: could not build $@
 	@echo built $@
 
-$(if $(findstring src/,$(MAKECMDGOALS)),$(MAKECMDGOALS), none): FORCE
-	$(MAKE) -C src $(patsubst src/%,%,$@)
-
 $(OSX_APP)/Contents/PkgInfo:
 	$(MKDIR_P) $(@D)
 	@echo "APPL????" > $@