Revert "kv: use origin timestamp to validate lww semantics"

This reverts commit 9336d4f.

Author: jeffswenson

pkg/kv/BUILD.bazel

@@ -52,7 +52,6 @@ go_test(
         "admission_test.go",
         "client_test.go",
         "db_test.go",
-        "kv_client_lww_test.go",
         "main_test.go",
         "range_lookup_test.go",
         "txn_external_test.go",