ANDROID: Delete unused protected_exports file

* The file gki/aarch64/protected_exports is no longer in use as there is
  a new mechanism in place, so delete it and update the docs
  accordingly.

Bug: 393366754
Change-Id: Ia6e512ce9f22f7f7acbd4ed423572c90030c9e37
Signed-off-by: Ulises Mendez Martinez <umendez@google.com>