Remove the Find Files example

The example doesn't do anything particularly interesting or teach-worthy, and uses outdated (and bad) patterns.

For instance, it scans text files matching a regexp in a for loop, and calls processEvents to keep the UI responsive, instead of performing the task in a thread and emitting signals to the UI.

The example's saving grace is that it's documented, but even that is no longer rendered correctly.

FWIW, some of this functionality could perhaps be folded into the general "document viewer" example. For the time being, we should move it into manual tests.