Remove KEYS file

http://mail-archives.apache.org/mod_mbox/incubator-general/201606.mbox/%3CCAAS6=7hVLcw6060Un7sXxk+WLLh08DFOSWktC0Aam4F=DyE0xA@mail.gmail.com%3E

> Bundling PGP keys inside a package is worse than worthless – an attacker can
just bundle spoofed keys with a bogus distro! Keys need to be made available
from a highly reliable, separate server: Download the main package from a
mirror, get PGP keys from apache.org, pgp.mit.edu, etc. and verify.
>
> The KEYS file within the Beam source tree should be deleted.