Remove AbortController test mock (#3506)

This library includes an `AbortController` test mock, which doesn't work
in places that expect a real one. For example, I was writing an
unrelated test that tried to cancel a `fetch()` which failed because
the `AbortController`'s `signal` property was invalid.

Because [`AbortController` is in all supported Node versions][0], we can
safely remove this mock and rely on the real thing.

This should only affect tests.

[0]: https://developer.mozilla.org/en-US/docs/Web/API/AbortController#browser_compatibility

setup-jest.js

@@ -206,9 +206,3 @@ NativeModules.RNMBXChangeLineOffsetsShapeAnimatorModule = {
 NativeModules.RNMBXLogging = nativeModule({
   setLogLevel: jest.fn(),
 });
-
-// Mock for global AbortController
-global.AbortController = class {
-  signal = 'test-signal';
-  abort = jest.fn();
-};