[lldb] [NFC] Remove unused WatchpointResource::SetID method (#79389)

I originally thought to try to guesstimate the hardware watchpoint index
number that a Resource was associated with, but gdb remote serial
protocol doesn't give us the hardware register index used so it was only
a guess. I changed my mind and simply use ever-incrementing ID numbers
for the WatchpointResources, but forgot to remove the SetID method.