Removed Kokoro config for 64-bit Linux

This Kokoro configuration is no longer being used and the build.sh
script does not work anymore, so let's delete this configuration.

kokoro/linux/64-bit/build.sh

-#!/bin/bash
-#
-# This is the top-level script we give to Kokoro as the entry point for
-# running the "pull request" project:
-#
-# This script selects a specific Dockerfile (for building a Docker image) and
-# a script to run inside that image.  Then we delegate to the general
-# build_and_run_docker.sh script.
-
-# Change to repo root
-cd $(dirname $0)/../../..
-
-export DOCKERFILE_DIR=kokoro/linux/64-bit
-export DOCKER_RUN_SCRIPT=kokoro/linux/pull_request_in_docker.sh
-export OUTPUT_DIR=testoutput
-export TEST_SET="csharp java_jdk7 javanano_jdk7 java_oracle7 javanano_oracle7 python python_cpp ruby_all javascript golang php_all"
-./kokoro/linux/build_and_run_docker.sh

kokoro/linux/64-bit/continuous.cfg

-# Config file for running tests in Kokoro
-
-# Location of the build script in repository
-build_file: "protobuf/kokoro/linux/64-bit/build.sh"
-timeout_mins: 120
-
-action {
-  define_artifacts {
-    regex: "**/sponge_log.xml"
-  }
-}

kokoro/linux/64-bit/presubmit.cfg

-# Config file for running tests in Kokoro
-
-# Location of the build script in repository
-build_file: "protobuf/kokoro/linux/64-bit/build.sh"
-timeout_mins: 120
-
-action {
-  define_artifacts {
-    regex: "**/sponge_log.xml"
-  }
-}