Apply clippy::semicolon_if_nothing_returned (#2410)

This is for consistency: if a block returns `()`, and the last
expression of the block also returns `()`, then the final `;` can be
left out.

However, this is a little confusing (I was asked about this in a class
today) and it is inconsistent.

See

https://rust-lang.github.io/rust-clippy/master/index.html#/semicolon_if_nothing_returned
for details.

src/android/aidl/birthday_service/src/server.rs

@@ -29,5 +29,5 @@ fn main() {
     );
     binder::add_service(SERVICE_IDENTIFIER, birthday_service_binder.as_binder())
         .expect("Failed to register service");
-    binder::ProcessState::join_thread_pool()
+    binder::ProcessState::join_thread_pool();
 }

src/bare-metal/aps/examples/build.rs

@@ -23,5 +23,5 @@ fn main() {
         .file("entry.S")
         .file("exceptions.S")
         .file("idmap.S")
-        .compile("empty")
+        .compile("empty");
 }