makefile/browser-asmjs: Remove now unneeded dependency to "base"

Following 09868d5 (browser-asmjs: Update to depend on existing image from
"trzeci/emscripten"), browser-asmjs does NOT depend anymore on base image.

Makefile

@@ -39,7 +39,7 @@ test: base.test $(addsuffix .test,$(IMAGES))
 #
 # browser-asmjs
 #
-browser-asmjs: base browser-asmjs/Dockerfile.in common.docker common.debian
+browser-asmjs: browser-asmjs/Dockerfile.in common.docker common.debian
 	sed -e '/common.docker/ r common.docker' -e '/common.debian/ r common.debian' $@/Dockerfile.in > $@/Dockerfile
 	mkdir -p $@/imagefiles && cp -r imagefiles $@/
 	cp -r test browser-asmjs/