buildscripts, psm interop: Don't fail target if sub-target already failed

We configured TestGrid to file bug separately for each 
failed sub-target, if we still fail the main target, 
TestGrid will fail duplicate bugs.

The same change in core: 
https://github.com/grpc/grpc/pull/33222.

buildscripts/kokoro/psm-security.sh

@@ -180,9 +180,6 @@ main() {
     run_test $test || (( ++failed_tests ))
   done
   echo "Failed test suites: ${failed_tests}"
-  if (( failed_tests > 0 )); then
-    exit 1
-  fi
 }
 
 main "$@"

buildscripts/kokoro/xds_k8s_lb.sh

@@ -181,9 +181,6 @@ main() {
     run_test $test || (( ++failed_tests ))
   done
   echo "Failed test suites: ${failed_tests}"
-  if (( failed_tests > 0 )); then
-    exit 1
-  fi
 }
 
 main "$@"