remove unused local obj and simpilify comple code (#9052)

Summary:
This PR does not change code sematics, it just changes for:

1. local obj `nonmem_w` and `lfile` are unused
2. null check for `delete ptr` is unnecessary
3. use `unique_ptr::reset` instead of `release` + `delete`

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9052

Reviewed By: zhichao-cao

Differential Revision: D31801661

Pulled By: anand1976

fbshipit-source-id: 16a77d45da8c8833bf5bf3bce546bb3711b335df

db/db_impl/db_impl_write.cc

@@ -1756,8 +1756,6 @@ void DBImpl::NotifyOnMemTableSealed(ColumnFamilyData* /*cfd*/,
 // two_write_queues_ is true (This is to simplify the reasoning.)
 Status DBImpl::SwitchMemtable(ColumnFamilyData* cfd, WriteContext* context) {
   mutex_.AssertHeld();
-  WriteThread::Writer nonmem_w;
-  std::unique_ptr<WritableFile> lfile;
   log::Writer* new_log = nullptr;
   MemTable* new_mem = nullptr;
   IOStatus io_s;
@@ -1861,17 +1859,9 @@ Status DBImpl::SwitchMemtable(ColumnFamilyData* cfd, WriteContext* context) {
   if (!s.ok()) {
     // how do we fail if we're not creating new log?
     assert(creating_new_log);
-    if (new_mem) {
+    delete new_mem;
-      delete new_mem;
+    delete new_log;
-    }
+    context->superversion_context.new_superversion.reset();
-    if (new_log) {
-      delete new_log;
-    }
-    SuperVersion* new_superversion =
-        context->superversion_context.new_superversion.release();
-    if (new_superversion != nullptr) {
-      delete new_superversion;
-    }
     // We may have lost data from the WritableFileBuffer in-memory buffer for
     // the current log, so treat it as a fatal error and set bg_error
     if (!io_s.ok()) {