Remove "app2sd" build dependency

Remove "app2sd" build dependency

* app2sd library supports APIs to handle external storage

[Issue#]   LINUXWRT-998
[Problem]  "app2sd" package dependency is no longer necessary.
[Cause]    symlink'ing to external storage is managed by platform level now.
           For this reason, WRT does not need to use "app2sd" package.
[Solution] Deprecate "app2sd" usage
           - Remove "app2sd" dependency.
           - Remove "src/domain/prepare_external_storage.cpp/h" and usage

[Remarks]
- To launch apps installed in external storage, symlink'ing to external storage is mandatory.
- WRT used to create symlink during web app installation time to external storage feature,
  but this is no longer necessary.

[SCMRequest] N/A

Change-Id: Iab51f9cb7a6ab58b5bf0cb7ee2378cfaaac5ddb3