storage.Delete not supported on Android/iOS

[midu-SA]

Describe the bug:

This bug is related to bug "File-I/O on Android #2118"

In order to create a work around for bug 2118, I tried to delete the existing file before writing the new data to it.
The method "storage.Delete (uri)" produces on Android the error
"operation not supported for this URI"

To Reproduce:

Steps to reproduce the behaviour:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Screenshots:

Example code:

Device (please complete the following information):

• OS: Android
• Version: Android 10
• Go version: 1.15.6
• Fyne version: Develop branch, pre 2.0.2