Fix removed structurally dead return statement

Summary:
There seems to be a typo mistake in env ReuseWritableFile func
where status is being returned twice.
Closes https://github.com/facebook/rocksdb/pull/3099

Differential Revision: D6196204

Pulled By: ajkr

fbshipit-source-id: abb6e3e1c1e772dd485fc39e7f1b9d502fa188fe

env/env_posix.cc

@@ -428,8 +428,6 @@ class PosixEnv : public Env {
       result->reset(new PosixWritableFile(fname, fd, no_mmap_writes_options));
     }
     return s;
-
-    return s;
   }
 
   virtual Status NewRandomRWFile(const std::string& fname,