Remove open-ended Android exercise (#2418)

Despite good intentions, I have not actually been able to make this
work in a classroom.

I suggest we remove it for now and later replace it with a guided
exercise: something where we ask students to write a AIDL client for a
simple system service.

src/exercises/android/morning.md

@@ -1,15 +0,0 @@
-# Exercises
-
-This is a group exercise: We will look at one of the projects you work with and
-try to integrate some Rust into it. Some suggestions:
-
-- Call your AIDL service with a client written in Rust.
-
-- Move a function from your project to Rust and call it.
-
-<details>
-
-No solution is provided here since this is open-ended: it relies on someone in
-the class having a piece of code which you can turn in to Rust on the fly.
-
-</details>