fix(settings): Prevent spurious Sentry errors from invalid data scrubbing selectors

[sentry]

The TraceItemFieldSelector.toField() method in static/app/views/settings/components/dataScrubbing/utils.tsx was unconditionally calling Sentry.captureException when it failed to parse a selector string. This meant that any invalid user input in the data scrubbing selector field, particularly during a blur event, would generate a Sentry error event instead of simply failing validation gracefully in the UI.

This change removes the Sentry.captureException call from toField(). Now, when toField() cannot parse a selector, it will quietly return null. The calling function, validateTraceItemFieldSelector, is already equipped to handle this null return by setting isValid: false and providing a user-facing error message, thus preventing unnecessary Sentry error reports for expected user input validation failures.

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Fixes JAVASCRIPT-3BNH

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