contrib: remove "examples" directory

The "examples" directory used to contain scripted versions of some of
our builtins. These have all been removed in 49eb8d3 (Remove
contrib/examples/*, 2018-03-25), but we left a note in the directory to
make it discoverable that there used to be examples.

It is unlikely that anybody still looks at these examples more than 7
years after they have been removed. Remove the note and its directory.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t