Fix Py3 breakage in new system test. (googleapis#5474)

Added in PR googleapis#5360.

Author: tseaver

bigtable/tests/system.py

@@ -446,7 +446,7 @@ def test_drop_by_prefix_table(self):
  read_rows_count = 0
 
  for row in read_rows:
- if row.row_key.decode('utf-8') in row_keys:
+ if row.row_key in row_keys:
  read_rows_count += 1
 
  self.assertEqual(expected_rows_count, read_rows_count)