Remove unneeded s3_crypto[.cc,.h] files

Both s3_crypto.cc and s3_crypto.h have been moved to
aws_crypto.cc and aws_crypto.h files. However, due to some
overlapping merge or update, s3_crypto.cc and s3_crypto.h
reappeared.

This fix removes unneeded s3_crypto.cc and s3_crypto.h files.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>