Remove unused variable - run_had_errors (#8599)

Summary:
Unused since https://github.com/facebook/rocksdb/commit/ab718b415fc9b2a66a2ed642c18803f764839d7b .
Noticed on https://lgtm.com/projects/g/facebook/rocksdb/snapshot/b215f1a83226f111ff52305987af93564272b7d3/files/tools/db_crashtest.py?sort=name&dir=ASC&mode=heatmap#xf254f528ad18f108:1

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8599

Reviewed By: ajkr

Differential Revision: D30057041

Pulled By: zhichao-cao

fbshipit-source-id: e80438cf9717086d2bf67461e19393d426a7676e

tools/db_crashtest.py

@@ -512,7 +512,6 @@ def blackbox_crash_main(args, unknown_args):
 
         for line in errs.split('\n'):
             if line != '' and  not line.startswith('WARNING'):
-                run_had_errors = True
                 print('stderr has error message:')
                 print('***' + line + '***')