chore(spanner): remove unused file (#3520)

Author: sakthivelmanii

.kokoro/readme.sh

@@ -88,7 +88,6 @@
     ".kokoro/presubmit/java8.cfg",
     ".kokoro/presubmit/linkage-monitor.cfg",
     ".kokoro/presubmit/lint.cfg",
-    ".kokoro/readme.sh",
     ".kokoro/release/bump_snapshot.cfg",
     ".kokoro/release/common.cfg",
     ".kokoro/release/common.sh",