fix: clear rawInput when removing repetition groups

[tkuhn]

Summary

• Fixes Placeholder values swapped when removing statement after form validation error #445: after a template form fails validation, removing a repetition group left the removed statement's placeholder value visible at the slot that took its place.
• Root cause: Wicket retains each FormComponent's rawInput after a failed validation and uses it for rendering instead of the model value. RepetitionGroup.remove() shifted model values down but didn't clear the stale rawInput, so the shifted-in value was masked.
• Fix: after each value swap (and the final clear) in remove(), walk context.getComponents() and call clearInput() on any FormComponent bound to the affected model.

Test plan

• Fill a template with a repeatable statement, create 3+ repetitions with distinct values
• Submit to trigger a validation error
• Click "−" on the first or middle repetition
• Confirm the values in the remaining slots correspond to the post-shift state (not the removed entry's value)
• Re-submit and verify nanopub contents match what is displayed

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