chore(spanner): revert precommit token check on mockspanner

Author: harshachinta

google-cloud-spanner/src/test/java/com/google/cloud/spanner/MockSpannerServiceImpl.java

@@ -2024,14 +2024,6 @@ public void commit(CommitRequest request, StreamObserver<CommitResponse> respons
       return;
     }
     sessionLastUsed.put(session.getName(), Instant.now());
-    if (session.getMultiplexed()
-        && !request.hasPrecommitToken()
-        && !request.hasSingleUseTransaction()) {
-      throw Status.INVALID_ARGUMENT
-          .withDescription(
-              "A Commit request for a read-write transaction on a multiplexed session must specify a precommit token.")
-          .asRuntimeException();
-    }
     try {
       commitExecutionTime.simulateExecutionTime(exceptions, stickyGlobalExceptions, freezeLock);
       // Find or start a transaction