fix: remove kokoro bigtable system tests (googleapis#8319)

# Description

https://togithub.com/googleapis/google-cloud-node/pull/8192 added the CI check to run Bigtable 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 anymore

Author: danieljbruce