rts: drop stale mentions of MIN_UPD_SIZE

We used to have MIN_UPD_SIZE macro that describes the minimum reserved
size for thunks, so that the thunk can be overwritten in place as
indirections or blackholes. However, this macro has not been actually
defined or used anywhere since a long time ago; StgThunkHeader already
reserves a padding word for this purpose. Hence this patch which drops
stale mentions of MIN_UPD_SIZE.