Fix the fix added in r343287 for spurious HFSC bandwidth check errors

The logic added in r343287 to avoid false-positive
sum-of-child-bandwidth check errors for HFSC queues has a bug in it
that causes the upperlimit service curve of an HFSC queue to be pulled
down to its parent's linkshare service curve if it happens to be above
it.

Upon further inspection/reflection, this generic
sum-of-child-bandwidths check does not need to be fixed for HFSC - it
needs to be skipped. For HFSC, the equivalent check is to ensure the
sum of child linkshare service curves are at or below the parent's
linkshare service curve, and this check is already being performed by
eval_pfqueue_hfsc().

This commit reverts the affected parts of r343287 and adds new logic
to skip the generic sum-of-child-bandwidths check for HFSC.

MFC after: 1 day
Sponsored by: RG Nets
Differential Revision: https://reviews.freebsd.org/D19124