PSA: app_shell has been removed from the bots/builders

[James Cook]

TL;DR: I've removed the deprecated binary "app_shell" (and its test suite "app_shell_unittests") from the CQ/CI builders and testers. If you see failures please ping me or revert CL, CL and  optionally CL.

app_shell was an experimental runtime for Chrome Apps. It was written ~10 years ago and never used in production. It supported extensions_browsertests, but that test suite was refactored away recently.

Our source tree is now 10K lines of code lighter.

Onward and upward,

James