Clean up Android DownloadManager code as most download now go through Chrome Network stack

The only exception is OMA DRM download.
And it only applies to context menu download interception.
Clean up the remaining unused code now.

BUG=647755

Review-Url: https://codereview.chromium.org/2371773003
Cr-Commit-Position: refs/heads/master@{#421332}
bool RegisterChromeDownloadDelegate(JNIEnv* env) {
  return RegisterNativesImpl(env);
}

bool RegisterChromeDownloadDelegate(JNIEnv* env) {
  return RegisterNativesImpl(env);
}
