cleanup: removes unused kokoro config files (#190)

This PR was generated using Autosynth. 🌈

Synth log will be available here:
https://source.cloud.google.com/results/invocations/afc024f5-fe80-4f85-85a6-2ebee6534e1c/targets

- [ ] To automatically regenerate this PR, check this box.

Source-Link: googleapis/synthtool@4530cc6

synth.metadata

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-storage-nio.git",
-        "sha": "0ef14c1f64d0acfe15ae1485a104d779e321e5bc"
+        "sha": "af492b8068101727793e64e89da2778205fa08b6"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "dd230c816f88d0141fcd0be83498986287220d1b"
+        "sha": "4530cc6ff080ef8aca258c1ec92c4db10a1bbfb4"
       }
     }
   ],
@@ -30,27 +30,16 @@
     ".kokoro/common.cfg",
     ".kokoro/common.sh",
     ".kokoro/continuous/common.cfg",
-    ".kokoro/continuous/dependencies.cfg",
-    ".kokoro/continuous/integration.cfg",
-    ".kokoro/continuous/java11.cfg",
-    ".kokoro/continuous/java7.cfg",
-    ".kokoro/continuous/java8-osx.cfg",
-    ".kokoro/continuous/java8-win.cfg",
     ".kokoro/continuous/java8.cfg",
-    ".kokoro/continuous/lint.cfg",
-    ".kokoro/continuous/propose_release.cfg",
-    ".kokoro/continuous/samples.cfg",
     ".kokoro/dependencies.sh",
     ".kokoro/linkage-monitor.sh",
     ".kokoro/nightly/common.cfg",
-    ".kokoro/nightly/dependencies.cfg",
     ".kokoro/nightly/integration.cfg",
     ".kokoro/nightly/java11.cfg",
     ".kokoro/nightly/java7.cfg",
     ".kokoro/nightly/java8-osx.cfg",
     ".kokoro/nightly/java8-win.cfg",
     ".kokoro/nightly/java8.cfg",
-    ".kokoro/nightly/lint.cfg",
     ".kokoro/nightly/samples.cfg",
     ".kokoro/populate-secrets.sh",
     ".kokoro/presubmit/clirr.cfg",