Remove /sdcard hack for shell users.

Remove /sdcard hack for shell users.

Using a /storage/sdcard0 symlink and a tmpfs staging area was used
to enable "/sdcard" paths for both zygote processes and the shell
user, but it breaks on devices with both emulated and physical
external storage.

Specifically, vold can't mount a physical sdcard onto the primary
storage symlink, and zygote can't re-bind-mount when secondary
storage is remounted after dropping privileges.

Bug: 7094858, 7064600
Change-Id: Id91e3b3e521a3b3e1bb5358920adc66a0a43917f