refactor: drop unused dirty flag in Spreadsheet connector

The local dirty variable in updated() was assigned but never read. The
'dirty' branch is kept as a no-op (like 'api'/'theme') so the reactive
property doesn't fall through to the unsupported-property console.error.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: vursen

vaadin-spreadsheet-flow-parent/vaadin-spreadsheet-flow/src/main/resources/META-INF/resources/frontend/vaadin-spreadsheet/vaadin-spreadsheet.js

@@ -188,11 +188,9 @@ export class VaadinSpreadsheet extends LitElement {
     super.updated(_changedProperties);
     this._overlays.setAttribute('theme', this.getAttribute('theme'));
     let propNames = [];
-    let dirty = false;
     _changedProperties.forEach((oldValue, name) => {
       let newVal = this[name];
       if ('dirty' == name) {
-        dirty = true;
       } else if ('rowBufferSize' == name) {
         this.api.setRowBufferSize(newVal);
       } else if ('columnBufferSize' == name) {