Remove unused screenshot (#4224)

Marshall Bowers created 

This PR removes an unused screenshot from the `assets/` directory.

As a note, we should probably be careful about what kinds of extraneous
binary assets we're committing to the repo (especially ones not required
for Zed itself), as these can bloat the repo size.

Release Notes:

- N/A

Change summary

assets/screenshots/staff_usage_of_channels.png | 0 
1 file changed

Detailed changes