Remove lock files when clean up session files

[jiangwen365]

What kind of change does this PR introduce?

• bug fix
• feature
• docs update
• tests/coverage improvement
• refactoring
• other

What is the related issue number (starting with #)
#1855

What is the current behavior? (You can also link to an open issue here)
Lock files are not removed, even when expired session files are removed by the clean-up thread.

What is the new behavior (if this is a feature change)?
Remove the lock files that have no session files at the end of session clean-up.

Other Information:
It might be a dirty fix just to save admins' efforts to delete the huge amount of obsolete locking files outside of CherryPy.

Checklist:

• I think the code is well written
• I wrote good commit messages
• I have squashed related commits together after the changes have been approved
• Unit tests for the changes exist
• Integration tests for the changes exist (if applicable)
• I used the same coding conventions as the rest of the project
• The new code doesn't generate linter offenses
• Documentation reflects the changes
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences