docs: Remove mention of setallreadmarkers
This command has been removed. To mark all channels as read you can now
run `/input set_unread` instead (note that since this isn't a command
specific for wee-slack, it will move the unread markers for all buffers
to the bottom, including buffers not belonging to wee-slack).
@@ -176,11 +176,6 @@ Add a reaction to the nth last message. The number can be omitted and defaults t
3+:smile:
```
-Set all read markers to a specific time:
-```
-/slack setallreadmarkers (time in epoch)
-```
-
Upload a file to the current slack buffer:
```
/slack upload [file_path]
