test: remove kokoro system tests for spanner

[danieljbruce]

Description

#8707 added the CI check to run spanner 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 as it will not be used anymore now that the config files are gone
• trampoline_v2.sh example refers to test.sh instead since system-test.sh doesn't exist anymore

[gemini-code-assist]

Code Review

This pull request removes several Kokoro configuration files and the .kokoro/system-test.sh script, while updating a reference in trampoline_v2.sh to point to .kokoro/test.sh instead. I have no feedback to provide as there are no review comments.