remove incorrect assert in GetUniqueIdFromFile (#5102)

Summary:
User report has shown that sometimes `BlockBasedTable::SetupCacheKeyPrefix` would assert when trying to generate an id from the file. The actual cause seems to be hardware related but we might be better off without the incorrect assertion
See T42178927 for more information
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5102

Differential Revision: D14604677

Pulled By: miasantreble

fbshipit-source-id: fcb09207ebdc4fa66e941afbc0523d84797e7ad7

env/io_posix.cc

@@ -265,7 +265,6 @@ size_t PosixHelper::GetUniqueIdFromFile(int fd, char* id, size_t max_size) {
 
   struct stat buf;
   int result = fstat(fd, &buf);
-  assert(result != -1);
   if (result == -1) {
     return 0;
   }