Simplify Kokoro configuration.

PiperOrigin-RevId: 206401009
Change-Id: I26644d1fe637b5a40db013fedf9fc063cc87ce6a

kokoro/continuous.cfg

@@ -1,7 +1,7 @@
 # Location of bash script that runs the test.  The first directory in the path
 # is the directory where Kokoro will check out the repo.  The rest is the path
 # is the path to the test script.
-build_file: "repo/kokoro/gcp_ubuntu/run_tests.sh"
+build_file: "repo/kokoro/run_tests.sh"
 
 action {
   define_artifacts {

kokoro/presubmit.cfg

@@ -1,7 +1,7 @@
 # Location of bash script that runs the test.  The first directory in the path
 # is the directory where Kokoro will check out the repo.  The rest is the path
 # is the path to the test script.
-build_file: "repo/kokoro/gcp_ubuntu/run_tests.sh"
+build_file: "repo/kokoro/run_tests.sh"
 
 action {
   define_artifacts {

kokoro/release-nightly.cfg

@@ -1,5 +1,5 @@
 # Location of bash script that builds a release.
-build_file: "repo/kokoro/gcp_ubuntu/run_build.sh"
+build_file: "repo/kokoro/run_build.sh"
 
 action {
   # Upload runsc binary and its checksum. It may be in multiple paths, so we