app: remove outdated TODO of app termination
app: remove outdated TODO of app termination

This was resolved in CL 11733. The "C.stopApp()" triggers the
termination of an app.

Change-Id: I48d4b798e42bc0c8b780ed5c0ec50968b62d0afa
Reviewed-on: https://go-review.googlesource.com/c/mobile/+/369199
Reviewed-by: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>
Reviewed-by: Emmanuel Odeke <bd234ba4276433f0e5fc7a8fa2d18274fa711567@orijtech.com>

golang/mobile,golang/mobile,golang/mobile,golang/mobile