Items removed from source storage are still present in list of tasks after syncing.

**Describe the bug**
I am labelling sound events from a list of audio stored in a local folder. My source storage is this folder and I sync it every now and then whenever I have new audio in it. BUT, when I delete some audio from this folder, and that I sync it in LS, the removed audio are not deleted from my list of tasks. I then have to remove them all manually or start a new project from scratch to have everything clean.

**To Reproduce**
Steps to reproduce the behavior:
1. Set a local folder as a source storage
2. Add some audio to it
3. Sync it in LS
4. Remove a couple of audio from this source storage
5. Sync it again in LS
6. Check the list of tasks (removed audio are still present)

**Expected behavior**
I want the removed audio to disappear from the list of tasks after I sync. Sometimes I just change the naming convention for my data and I end up with two versions of the same data in my list of tasks, with different names, because the old one didn't get removed.

**Screenshots**
N/A

**Environment (please complete the following information):**
- OS: Windows 10
- Label Studio Version 1.7.3dev

**Additional context**
N/A