data store: catch possible traceback

[oliver-sanders]

• Closes traceback: remove(x): x not in container  cylc-uiserver#473

I'm not sure what circumstances can cause this, seems sensible to ignore KeyError i.e. the thing you were trying to remove wasn't there, @dwsutherland any thoughts?

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• CHANGES.md entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[dwsutherland]

Makes sense, even for the UIS..