Searchbox Enter is to clear, tabbing to clear button doesn't work, remove it

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

src/components/structures/SearchBox.js

@@ -133,7 +133,9 @@ module.exports = createReactClass({
             return null;
         }
         const clearButton = (!this.state.blurred || this.state.searchTerm) ?
-            (<AccessibleButton key="button"
+            (<AccessibleButton
+                key="button"
+                tabIndex={-1}
                 className="mx_SearchBox_closeButton"
                 onClick={ () => {this._clearSearch("button"); } }>
             </AccessibleButton>) : undefined;