TEZ-4532: Remove KEYS file from git repo and source distribution

Projects must publish a KEYS file in their distribution directory which contains all public keys used to sign artifacts (i.e., https://dist.apache.org/repos/dist/release/tez/KEYS).

The KEYS file here is not serving any purpose since it shouldn't be used to verify releases. It adds maintenance overhead (assuming that is kept in sync) and possible confusion to the users with respect to which file they should use to validate the artifacts.