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/SUMMARY.md

@@ -258,7 +258,6 @@
     - [Building for Android: Genrules](android/interoperability/cpp/android-cpp-genrules.md)
     - [Building for Android: Rust](android/interoperability/cpp/android-build-rust.md)
   - [With Java](android/interoperability/java.md)
-- [Exercises](exercises/android/morning.md)
 
 # Chromium
 

src/android.md

@@ -9,11 +9,6 @@ Rust is supported for system software on Android. This means that you can write
 new services, libraries, drivers or even firmware in Rust (or improve existing
 code as needed).
 
-> We will attempt to call Rust from one of your own projects today. So try to
-> find a little corner of your code base where we can move some lines of code to
-> Rust. The fewer dependencies and "exotic" types the better. Something that
-> parses some raw bytes would be ideal.
-
 <details>
 
 The speaker may mention any of the following given the increased use of Rust in

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>