test: remove kokoro system tests for error-reporting

[danieljbruce]

Description

#8699 added the CI check to run error-reporting system tests in GCB so now we don't need to run the system tests in kokoro anymore. This pull request removes the CI check to run system tests from the kokoro environment because it is now redundant.

Impact

Removes redundant testing to save resources.

Files changed

• Removes the system-test.cfg files so that kokoro doesn't try to run system-test.sh from continuous or presubmit environments.
• Removes the system-test.sh file and presystem-test.sh file as they will not be used anymore now that the config files are gone
• trampoline_v2.sh example refers to samples-test.sh instead since system-test.sh doesn't exist anymore

[gemini-code-assist]

Code Review

This pull request removes several Kokoro system test configuration and script files, and updates a comment in trampoline_v2.sh to reference test.sh instead of system-test.sh. There are no review comments, so I have no feedback to provide.