Don't set IFCAP_TXRTLMT during lagg_clone_create().

lagg_capabilities() will set the capability once interfaces supporting
the feature are added to the lagg. Setting it on a lagg without any
interfaces is pointless as the if_snd_tag_alloc call will always fail
in that case.

Reviewed by: hselasky, gallatin
MFC after: 2 weeks
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D19040