Do you get an error message when you try to delete?
I'm assuming so since you're trying to modify a system table.
You can modify system tables in Yukon if you log in using the DAC.
So you'll connect like this:
That should allow you to modify the system table. If not, then you may have to throw the system into single user mode first... I can't remember, but I don't think you should have to do that.
Anyway, failing anything else, you can always move msdb to a diff. partition.