test: remove kokoro system tests for datastore

[danieljbruce]

Description

#8343 added the CI check to run datastore 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 samples-test.sh instead since system-test.sh doesn't exist anymore
• owlbot.py doesn't need to pay any attention to system-test.sh file and other removed files anymore

[gemini-code-assist]

Code Review

This pull request removes Kokoro system test configurations and scripts, and updates owlbot.py to exclude system-test.sh from being copied. The reviewer recommends also adding the deleted system-test.cfg files to the excludes list in owlbot.py to prevent OwlBot from recreating them on future runs.