More code review updates

Author: odeke-em

google-cloud-spanner/src/main/java/com/google/cloud/spanner/XGoogSpannerRequestId.java

@@ -159,26 +159,4 @@ public XGoogSpannerRequestId nextRequestId(long channelId, int attempt) {
       return XGoogSpannerRequestId.of(1, 1, 1, 0);
     }
   }
-
-  static void assertMonotonicityOfIds(String prefix, List<XGoogSpannerRequestId> reqIds) {
-    int size = reqIds.size();
-
-    List<String> violations = new ArrayList<>();
-    for (int i = 1; i < size; i++) {
-      XGoogSpannerRequestId prev = reqIds.get(i - 1);
-      XGoogSpannerRequestId curr = reqIds.get(i);
-      if (prev.isGreaterThan(curr)) {
-        violations.add(String.format("#%d(%s) > #%d(%s)", i - 1, prev, i, curr));
-      }
-    }
-
-    if (violations.isEmpty()) {
-      return;
-    }
-
-    throw new IllegalStateException(
-        prefix
-            + " monotonicity violation:"
-            + String.join("\n\t", violations.toArray(new String[0])));
-  }
 }