Remove file exist assertion

Assertion will not compile in production and assert file existence is not much useful in lab environment. Moreover it is not easy for testing.

Author: evan361425

lib/src/cropper.dart

@@ -76,7 +76,6 @@ class ImageCropper {
     AndroidUiSettings? androidUiSettings,
     IOSUiSettings? iosUiSettings,
   }) async {
-    assert(await File(sourcePath).exists());
     assert(maxWidth == null || maxWidth > 0);
     assert(maxHeight == null || maxHeight > 0);
     assert(compressQuality >= 0 && compressQuality <= 100);