only run system test in Kokoro presubmit; others run via github actions

Author: parthea

noxfile.py

@@ -72,15 +72,7 @@
 CURRENT_DIRECTORY = pathlib.Path(__file__).parent.absolute()
 
 nox.options.sessions = [
-    "unit",
     "system",
-    "cover",
-    "lint",
-    "lint_setup_py",
-    "blacken",
-    "docs",
-    "docfx",
-    "format",
 ]
 
 # Error if a python version is missing