Auto merge of #26249 - steveklabnik:remove_extern_panic, r=alexcrichton

This test has an interesting history, because of fail -> panic. It was
originally called extern-fail.rs:
https://github.com/rust-lang/rust/commits/master/src/test/run-fail/extern-fail.rs

It lived for a while, but was disabled in August 2013:
https://github.com/rust-lang/rust/commit/ce95b01014391f29a655d165d9e6d31449ceb835

As you can see, that failure was not specific to this test, however,
this code does stuff with the runtime, which was removed. Given that
it hasn't even been able to compile in a long time, we should just
remove it.

r? @brson