more SvStream:operator>> conversion

Convert the template based read_lenPrefixed methods to regular
methods.

Change-Id: Ifd0e93aca055e55a0575e4377ec2b8e266dfb019
Reviewed-on: https://gerrit.libreoffice.org/7895
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>