Remove redundant recursion quota pointer checks

Remove redundant recursion quota pointer checks

When the client->recursionquota pointer was overloaded by different
features, each of those features had to be aware of that fact and handle
any updates of that pointer gracefully.  Example: prefetch code
initiates recursion, attaching to client->recursionquota, then query
processing restarts due to a CNAME being encountered, then that CNAME is
not found in the cache, so another recursion is triggered, but
client->recursionquota is already attached to; even though it is not
CNAME chaining code that attached to that pointer, that code still has
to handle such a situation gracefully.

However, all features that can initiate recursion have now been updated
to use separate slots in the 'recursions' array, so keeping the old
checks in place means masking future programming errors that could
otherwise be caught - and should be caught because each feature needs to
properly maintain its own quota reference.

Remove outdated recursion quota pointer checks to enable the assertions
in isc_quota_*() functions to detect programming errors in code paths
that can start recursion.  Remove an outdated comment to prevent
confusion.