Fix flaky test asserting concurrent write thread's waiting time (#13241)

Summary:
This test assertion was added in #13219. It checks the concurrent write thread's wait time is not longer than the file ingestion thread's write blocking time since the former entered the write thread after the blocking already started in the test. This test runs into flakiness like this:
```db/external_sst_file_basic_test.cc:300: Failure
Expected: (perf_context.file_ingestion_blocking_live_writes_nanos) > (write_thread_perf_context->write_thread_wait_nanos), actual: 166210 vs 279681
```
 In reality the write thread is yielding starting with a 1 micro period and then every 100 micros: https://github.com/facebook/rocksdb/blob/54b614de5bd3e26d332b85557d44bde86b2a2e87/db/write_thread.cc#L68-L70

So this 113 micros errors is within this margin
This fix the test with just removing this assertion. The other assertion `ASSERT_GT(write_thread_perf_context->write_thread_wait_nanos, 0)` should be sufficient for the test's purpose.

Pull Request resolved: #13241

Reviewed By: hx235

Differential Revision: D67526804

Pulled By: jowlyzhang

fbshipit-source-id: 23ee9771247e4c13444054a1e86ad9293902cb56

db/external_sst_file_basic_test.cc

@@ -297,8 +297,6 @@ TEST_F(ExternalSSTFileBasicTest, Basic) {
   }
 
   write_thread.join();
-  ASSERT_GT(perf_context.file_ingestion_blocking_live_writes_nanos,
-            write_thread_perf_context->write_thread_wait_nanos);
   SyncPoint::GetInstance()->DisableProcessing();
 
   // Re-ingest the file just to check the perf context not enabled at and below