Fix big grid crashing due to missing React import

by fixing the cause rather than the symptom: this upgrades the code to use the new, recommended JSX transform mode of React 17+, which no longer requires you to import React manually just to write JSX.

test/home/CallList-test.tsx

@@ -14,8 +14,6 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import React from "react";
-
 import { render, RenderResult } from "@testing-library/react";
 import { CallList } from "../../src/home/CallList";
 import { MatrixClient } from "matrix-js-sdk";