build: Remove unused script

* http_prebuilt.mk appears to be long gone.

Change-Id: I8ca584fc1e468bb47164984bb363f0cf29750e68

build/tasks/http_curl_prebuilt.sh

-#!/bin/bash
-
-url=$1
-output=$2
-
-curl -L "$url" --create-dirs -o $output --compressed -H "Accept-Encoding: gzip,deflate,sdch" && exit 0 || exit 1